The V-10 Triton from Ford with their 6 speed automatic is a good set up and the most widely available today in a gas powered coach. In days gone by the 8.1 GM and Allison were the best for a gas engine combination but their fuel mileage was not quite as good as the Ford usually. The V-10 will work for a normal sized towed load (read that an average car or trailer). Unless you are going to put a lot of miles on towing, there is no need to get a diesel powered coach for just that reason.
The V10 has served us well. Need to stay around 4k and less for toad imo. Tow rating on mine is 5k. 65 K so far and no problem with drivetrain. MPG averages 8 towing. Only traveled one time with no toad. Alabama to Alaska and back towing a 4 door 4x4 chev tracker. Now have a saturn vue for toad. I would not go over a 34 foot in the gasser. Over that diesel.