There's a lot more to the choice than "gas vs diesel" and probably more motorhomes to choose from than the two particular models you are considering.
First, how big does it need to be, to suit your needs? To some extent, the gas vs diesel choice is about size. Generally, diesel motorhomes are larger, so that choosing diesel means choosing larger, but there is some overlap, biggest gas motorhomes may be within a few hundred pounds of the smallest rear-engine diesel motorhomes.
The Ford motorhome chassis using the V-10 (E and F series) starts at 11,000 pounds maximum weight and 138" wheelbase, can be configured up to 12,000 pounds heavier and almost ten feet longer (wheelbase to 252"), carrying motorhomes from about 26 foot to 38 foot length. The engine is up front and the motorhome rides (sometimes harshly) on leaf springs. The F-53 chassis from Ford is nice for motorhomes 32 to 36 feet, up to about 22,000 pounds, but larger is pushing more at the limits of what can be carried.
The Freightliner XC chassis (the one that would have had the C-7 engine) is designed to carry larger motorhomes, 208" to 296" wheelbase, 26,000 to 34,000 pounds maximum weight. The engine is in the back where the noise is less intrusive, and the motorhome rides (usually smoothly) on air springs, like motorcoaches. The motorhomes on this size chassis are typically 34 to 40 foot long, though at 40 feet you are usually wanting a more powerful diesel engine.
Diesel motorhomes can be even larger, on other chassis series or built inside motorcoach shells, up to the maximum legal length of 45 foot, and weighing nearly 55,000 pounds. These carry a much more substantial house than can be built on a gas engine chassis that can't handle even half the weight.
So if what you want is a small motorhome (less than 34 foot), it is probably going to be a gas engine (today for a new one that means Ford, V-10 or in a really small one, V-8). Exception would be some newer small front-engine diesels about 25-26 feet long, under 12,000 pounds. If you want something 38 to 45 feet long, and over 12 tons, it will likely be a diesel. 34 to 40 feet, it can be either.
Tom Test
Itasca Spirit 29B