Loaded question. A stock 350 carries anywhere for 145HP to 600HP. If you have the right combination setup for low end, sure, a 350 is fine, but a 454 with a mild cam is better. I own a Durmax myself, and that is the answer, but probably not conducive to your application.
Your easiest solution would be to have the right engine builder get the right valvetrain setup for haulin for you for a 350. Probably the cheapest too. Next best bet, I'd shoot for a 454 with a cam setup.
I don't think an Duramax swap is realistic. It's drive by wire, fuel coolers, intercoolers, fuel tank swaps, etc. Too much money. Just sell the motorhome and buy something with a diesel if that's what you want.
If you're in the rockies, gearing makes a big difference as well without doing anything with the engine too.
