I would not worry about what fuel the motorhome uses or what powerplant it has from a performance standpoint. It's going to be rather on the sluggish side compared to a car no matter what, but likewise it will get you where you need to go regardless. Some may be a little quicker than others going up a mountain, but that's relatively little of your travel time.
Acceleration off the line depends on how much power you can get to the wheels relative to the weight of the motorhome. Many diesel pushers are heavier than gas motorhomes, and don't have correspondingly more power, so they are slower in initial flat-out acceleration. For climbing a hill, again weight vs. power comes into play, but the power also has to overcome air resistance which is probably closer to equivalent across motorhomes. At altitude still another difference comes into play since gas motorhomes generally are not turbocharged while diesels are, and so the gas one will see power drop off more due to the thinner air.
The net of everything is that a lightweight gas motorhome may well accelerate faster than some diesel motorhomes that have higher weight or lower power output, while other diesel motorhomes will outperform them. None of that matters one bit when you're sitting in a campsite eating s'mores.
That being said, I would avoid the old four cylinder Toyota pickup-based class C's if you care about vehicle performance at all.