There are lots of debatable points, several of which have already been mentioned. How much weight you give to any difference is often a matter of personal preference. Some may even classify as a CON what others consider a PRO.
The market's opinion is reflected in the price - the diesel pusher is worth the same as the much newer and lower mileage gas model. That's a summary of all factors but an important conclusion.
The diesel engine & fuel are a relatively small piece of the difference. Engine horsepower is about the same and diesel fuel no longer has a cost advantage vs gas like it did 20 years ago. Nowadays the transmissions are much alike as well, 6 or more gears, automatic downshifts, etc. The significant performance advantage of the diesel is largely that it produces its full horsepower at low engine rpms where it is most needed. Few drivers are ever willing to rev the gas engine up enough to deliver its max horsepower. The penalty for that diesel performance advantage is extra weight, higher initial cost, some additional maintenance expense, and potentially greater repair expense.
The rest of the DP mechanical advantages are in the chassis design choices. Heavier duty axles and steering, greater GVWR and GCWR, air suspension and brakes, more robust frame rails, bigger cooling system, etc. The combined effect is somewhat better ride and handling plus greater durability. However, that comes at a $$ cost, both up front and long term.
It has been suggested that gas chassis have much improved over the past 15 years and the gap between gas & diesel motorhome chassis has narrowed. I think that is unequivocally true, but there is still a gap. The superior capabilities of the more expensive mechanical components found in most DP chassis (as mentioned above) still yield a better overall experience. A gas engine chassis with those same attributes could be built, of course, but if you are going to upgrade all the rest of the chassis, why not upgrade the engine as well? It comes down to cost. A gas engine on 22,000 lb frame rails plus spring suspension and hydraulic brakes performs decently at a price that can be swallowed by many potential buyers. The somewhat better and somewhat more expensive DP chassis narrows the field to more affluent buyers.
Once the decision is made to build on the more expensive DP chassis and market to more affluent buyers, it's an easy step for the coach builder to also select better but more expensive cabinetry, furnishings, upholstery, etc. The buyers in that price tier expect more and are willing to pay for it, so generally the DP models will be better built. There is still a range from low end to high, but the starting point is near always higher.
The practical effect is that an entry level diesel pusher (DP) is typically built to be equivalent to a high end gas model. The price is moderately higher and usually a few extra features are included to give more visible value. Then the rest of the DP models move upscale from there.