asilverblazer

Missing airbags without a warning light seems suspect to me.

I think the easy thing is to unplug the headlights and mirrors. However, I'm inclined to say they are unrelated (unless there was an installation issue that damaged wires). I would suggest checking ground integrity. Clean cables, posts, attachments and make sure the cables are not corroded inside the insulation. Check fuses and inspect for other wiring issues. Make sure all the connectors at the PCM are connected well without corrosion.

Check the trailer running gear, maybe a hub seizing up, dragging brake shoe. Could apply to the truck too...

It sounds to me like you are describing bumps and imperfections that are too small to compress the springs and subsequently too small for the shock absorbers too have any impact as well. Your basketball dribbling is a very apt description - what is a tire if nothing vastly different from an air filled rubber ball? Notice the difference in how a basketballs bounce changes based on the amount of air in it? If you've lowered the air pressure and can't get the dribbling under control that way, the next suggestion would be to try different tires (and wheels with a smaller diameter). Despite my comments above a softer shock may help some if the dribbling is sufficient to compress the springs. Lastly it could be a combination of shocks, tires, wheels are needed to get the ride quality you want.

You have to better describe bouncy. Does the truck continue up and down motion after hitting a bump, similar to a boat riding up and down waves? No individual impact is noticed. -OR- Does the truck hit a bump and experience one harsh hit without continual motion? Every little impact is felt as harsh hits and jars. Easy things to check: Tire pressure, no higher than the door sticker. Easy things to change: Shock absorbers, there is a bunch of talk about these but you first need to determine what type of suspension motion (spring action) you are trying to adjust.

Serpentine belt?

I'd say unrelated except the increased engine load is inducing the miss fire. Without the ability to actually diagnose in person, my guess is a bad injector, can't keep up with fuel delivery under load. How did the mechanic(s) confirm all the above suggestions were not the problem? All other possibilities are more and more unlikely.

Was the check just shoe life or drum roundness? Its not the ABS kicking in is it?

I would get a subscription to alldata, it will provide a diagnostics of the circuit. I don't have one for that model so can't offer more assistance.

NEW brakes + NEW noise = check the work that was done. Leaf spring bushings do not grind they clunk.

Even with adding the additional modules, the switch, the wiring additions (which will be significant and likely insurmountable for those that are thinking just add the switch and go) there is still the programming that will likely be impossible to resolve. To date - this is still IMPOSSIBLE.

Look again? https://www.gmupfitter.com/wp-content/uploads/2021/05/2012_LD_ElectricalPickupsChassisCabs_100813.pdf https://www.gmupfitter.com/wp-content/uploads/2021/07/2013_LD_ElectricalPickupsChassisCabs_100713.pdf https://www.gmupfitter.com/wp-content/uploads/2021/05/2006_LD_CKFullSizeElectrical.pdf https://www.gmupfitter.com/wp-content/uploads/2021/05/2011_LD_ElectricalPickupsChassisCabs_100813.pdf

I hope the OP has figured it out by now.

Check the wiring and terminals to the sensor. What is everything?

Example: Intermittent miss cylinder #7, this only occurred on 'wetter' days. Visual inspection seemed fine, a new cap fixed it. Also, found that the cheaper caps terminals would corrode on the inside causing intermittent miss fires. Pop the cap and clean the corrosion and miss went away. I also have had a brand new set of plug wires with one bad wire straight out of the box. Or, you can start looking at internal engine components that are specific to cylinder 3 that are more expensive to inspect and replace and less likely to be intermittent or load based. Cam shaft lobes Push rods Lifters Valves Rockers Intake gasket Head gasket An intermittent miss fire with no other symptoms such as smoking, noises, other codes, etc. doesn't indicate an internal failure to me. Fuel delivery up to injector 3 is not the problem... Ignition up to distributor cap terminal 3 is not the problem... Air intake up to intake valve 3 is not the problem... Sensors that monitor the entire engine or cylinder bank are not the problem... ... otherwise the problem would be more widespread.

Everyone knows the funnest car game is seeing how much you can beat the GPS ETA.

You need to figure out what is wrong with cylinder 3 and only cylinder 3. Things that are unique to that cylinder: Plug Wire Distributor Cap Terminal Injector Valves New parts does not equal good parts. Try swapping the plug wire then plug to known good cylinders to see if the miss follows one of those parts. Thoroughly and carefully inspect the cap, I noted earlier that my experience with this generation of cap is not good. Especially with intermittent miss fires like this. make sure it is a good quality part (AC/Delco and not a counterfeit one at that). Inspect wiring to the injector. MAF sensor, O2 sensors, MAP sensor, fuel filter, fuel pump and the like will not cause a miss on only one cylinder. Considering the head has been off and the valves have been worked on, confirm the compression is good - If this were a problem I would expect a dead miss, not intermittent though.

MAF sensor is delicate, hope you didn't mess it up cleaning it. A poor running engine won't get good gas mileage, sounds like it isn't fixed yet. Parts cannon approach can cause more problems than it solves. You may now have two problems, the original miss fire plus a potentially damaged MAF sensor. (Not trying to criticize.) Proceed with caution, first, try data logging if you can to see if some cylinders are miss firing more than others.

Have heard no mention of distance and terrain you'll be towing in. Properly setting up the weight distribution and sway control will go a long ways to making the set up feasible. It is not about the overall weight in a travel trailer, its the size, followed by tongue weight. Tongue weight will end up maxing out the GVWR of the truck first. Tire size on the truck is probably the biggest down side, followed by the lift. The first thing you will run into towing a travel trailer is the engine RPM's will be high - all the time. MPG's will be low 8-9. Your lift may actually help setting up the weight distribution shank because they are usually too low for a 1/2 ton. Overall the lift is going to be a negative though for stability and aerodynamics. If you are traveling less than 1000 miles, not in severe terrain and no faster than 70mph you could probably make it work. I was able to manage a 35' (actual box measurement) travel trailer behind my 1/2 ton. Be on the look out for all the "I'm gonna sue you if you crash into me and you're over weight people".

Still have NEVER seen a failed PCM Maybe Its either ignition or fuel...

My experience with that generation always had issues at the distributor cap. I've also had brand new plug wires that one was bad. Presume that cylinder 3 (P0303) is the main problem. See if swapping plug wires from that cylinder to another moves the miss to the swapped cylinder. Take a look at this thread:

Bummer.

If you have a scan tool with live data, you could probably watch for it.

Find the fuse for the compressor and pull it. That'll keep it from running.

I am still curious about the miss only occurring after the engine has warmed up. Have you confirmed whether it begins at switching to closed loop operation or just when things start getting hot?