It's all just common sense. Auto (engine) manufactures aren't going to let you play with the programming of their engines and stand behind the warranty. They would be idiots if they did. Yes, some dealers look the other way. Most don't , and "most" of the time they know if you have played with it or added a tuner or chip.
Bottom line if you have a big wallet and want to make you engine more powerful then it's your money as your going to flip the bill. If not then its wise to leave the engine alone while under warranty, or you might not be happy when something breaks.
Why not just get the truck with a engine capable of doing the job stock from the factory? With the internet there are plenty of ways to find out how engines perform while towing or in MH's well before you buy one. Being informed before you spend the money on one can help you with wishing it was just a little better. For most going from a gas engine to a diesel solves most of the power issues. And there are many diesel MH platforms that can supply you with the needed power, even if you don't need it.