Forget the "gas is faster" theory - that's old news from the pre-turbo days. Nowadays the kids buy diesel trucks because they outperform gas engines. A turbo-charged diesel has so much more low end torque than a gas engine that they beat the pants of all but the largest gas engines form a standing start. And once they are rolling, they can accelerate with the best of them too. But they run out of top end speed sooner because of the lower RPM range.
If money is no object, choose the diesel, whether truck or motorhome. And if you want to tow a trailer with a truck, go diesel. ON the motorhome end, gas vs diesel is a cost/benefit tradeoff. A diesel pusher chassis is more than just a question of diesel vs gas - you get air suspension, rear engine quietness, greater weight capacity, an industrial grade transmission, etc. You pay more, you get more and well yu sell it you get more back. But the upfront cost differential is a biggie.
A modern gas chassis motorhome performs well and is very satisfying to own and drive, but the diesel pushers are better. Enough better to justify the price? Only you can answer that.
By the way, there are front engine diesels (called FREDs - FRont Engine Diesels) and a rear engine gas chassis has just come out (Workhorse UFO) but I haven't seen a motorhome built on it yet.
