The first question you must answer to yourself is what size of coach you are looking to get. If it is a smaller one, most likely you will need to get a gas coach. If a larger coach, then diesel. In the middle you have the choice of gas or diesel.
I am a rare one in that I converted my gas pusher coach to a diesel pusher coach. I much prefer the diesel for torque at lower engine rpm which means quieter and better fuel mileage (the coach stayed the same weight!). I also prefered the pusher configeration when my coach was gas, but there was only a few of those made for a few years in recent time(the UFO chassis).
Really look at the size and configuration before you look at the power plant.