SVG

I'll second @Anacortes Army Guy in using a second trailer. Or a second truck to hook up to your trailer. I had the same issue with a brand new boat trailer. My Ol' 96' Dodge would work the brakes and lights but my then new 21' GMC wouldn't actuate the EOH brakes?? After much hair pulling I found out about a HBA-CAM module. When I called their manufacturer they were aware of the issue. Since installed my GMC works like it should. Might be worth a phone call. Hydrastar HBA-CAM Installation Sheet.pdf

I like it!

I would take it back to the dealer and make them get it right. When they finally get it right, or near right, I'd trade that sucker in on a new rig. Even though I am very happy with my current GMC truck, if I felt abused by GM like you've been, I would probably be looking at a different manufacturer. Or you could take a less emotional stance and realize all manufacturers spit out a lemon every now and then. Honestly, with the amount of miles you put on a vehicle, I'd take a look at a refreshed 3.0 Duramax.

When I was shopping trucks last year (Ford & GMC, no Dodge, I already have one) the massaging seats in the Ford almost made me get that truck. In the end the 3.0/10 speed combo won me over to GMC (yes I know Ford has a 10 speed as well).

OP, I had a custom welded aluminum boat trailer built last year. My AT4 wouldn't actuate the EOH brakes. After much pulling of hair (that is a diminishing asset) I found that some truck's don't play well with some brake systems. Once I added an HBA-CAM to my boat trailer all things worked as they should. If you have electric only brakes than this may be of no use to you. HBA-CAM .pdf

The new interior looks great!

I pull my ~5,000 Lb. boat with zero drama with my AT-4. I have 20" wheels with Goodyear Territories. Don't know how much difference they make but I'm very happy with the towing experience. I've also got a bagged 3/4 ton 12V Cummins for the heavy lifting. For a daily driver the GMC is much more comfortable.

Sounds to me like your seal blows when are passing somebody. Maybe drive in the slow lane?? Just kidding OP. Hope you can find out what's causing this.

None whatsoever in my black paint (I think they call it Ebony Twilight).

I'm in NW MT where it gets cold too. Not one issue with my 3.0 in it.

I'd go for a refreshed 3.0. Take one for a test drive if you can find one. It feels like the truck is hardly working. Been very happy with mine and would get another in a heartbeat if something happened to it.

Thanks. Loving the truck so far. I dealt with Mike as well. Good guy.

I know my 12V Cummins has a fuel return line back to the tank so that surely helps the mixing issue. Do our 3.0's have a return line as well?

Nathan is spot on about Robideaux Motors in Coeur d'Alene. I drove from Montana to pick up my AT4. They had what I wanted in stock (it had arrived the afternoon before) and had excellent pricing as well. Not trying to make this sound like an infomercial but RM was great to deal with. I'll attach a picture of the day I picked up my truck. I was parked at a little roadside Mexican food place in CDA (killer burritos!).

I'll echo @seamus2154 's comments about the 3.0. It's a winner in my book. A for 2 wheel drive vs. 4 wheel drive. Resale is usually better with 4WD. FWIW