If you are pulling an enclosed trailer go for the diesel. I've had two class A motorhomes with the big blocks. Five transmissions in the Dodge powered Winnie and on my second in the Chevy powered Southwind. Motors have held up but are gutless with all the weight. Milage always sucks but you've got a cheap hotel at the track. If you can find a diesel with Allison trans, that's the ticket. Pusher bus is even better.
Also - very important - as Ben said, get the chassis reinforced for the tongue weight on the hitch.
Chuck