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eng going into Limp mode, HELP

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So, folks I need some help, we have a 2005 Silverado 3500 DRW LB 4x4 with the LLY Eng. We are full time RVers and have had no problems the last 2 yrs. towing a 15,000# worth of 5th wheel. Well I have had the truck go into what I think is the “limp mode” from what I can tell based on some quick internet searching 3 times the last month or so while towing.  First time was after about an hour of towing over to the coast and on the last hill after starting up it happened until we got over (small hill, maybe 7 or 800 Ft elevation) and then it ran normal the last 2 miles to the rv park.  The 2nd time we were not towing but again going up a slight incline but this time it only lasted about 5 min than back to normal driving. The 3rd time was about 4 or 5 min into towing and just as we started a very slight incline but it stayed in that for the whole 10-mile trip to go pump out.  After being shut off while pumping it ran normal for 3 or 4 min on the trip back then back into limp.   Ok the truck has run normal for well over 300 – 400 miles in-between these events at all speeds, normal sounding and feeling shifts, up or down inclines.   It will run ok up to about 28mph and maybe 2500 to 2600 RPMs at about half throttle.  If I try to give it more it starts to surge / buck. I can’t get any more speed or RPMs but if I back off the gas it returns to fairly normal but at these limits.  There has been no overrev like the tranny is slipping or anything.  I had tranny fully serviced about 15,000 miles ago and its full of good-looking fluid. I changed the fuel filter and it looked good as well. I have looked in the eng compartment and find no obvious shorted wiring.  I am going to take it to a shop I know and see if it showing any error codes but I’m not real hopeful that’s going to show much if anything.   So, any Ides as to other things I can look at / for.  I really can’t afford an expensive repair at the min. We can sit where we are for the time being here in Oregon as the truck seems to do fine if I’m not towing.  But Yuma would be a much warmer location...  LOL

 Any thoughts or advice are very much appreciated.

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You might check the egr, I’m new to my duramax, but my wife’s x5 diesel had a similar intermittent limp mode, I cleaned the egr valve up, and later found a boost leak into the manifold, haven’t seen it since we fixed both of those. I paid the dealer to fix the boost leak, which might produce similar symptoms

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