Take it to a COMMERCIAL TRUCK repair place and have them look at the power-train, chassis and brake system because you own a truck. The rest you already know how to do (fridge, water system, heat, roof a/c. While there have an engine oil and transmission oil sample taken and mail it to http://www.blackstone-labs.com/. in the kit that Blackstone will mail you if you ask. The basic tests are $28.00 each
They will be able to tell you what is going on in the engine and transmission that the mechanic cannot see. Have the truck place do a compression test and a radiator pressure test.
Make sure the place you take it is happy to see you and is not doing it grudgingly then you have found your go to place for repair. I feel myself blessed that I have 2 places like that in my life one in NYS and the other in Arizona