If I read you right, you want to tow a trailer that can carry both an ATV and a car. That is almost surely going to exceed the tow capacity of a gas-chassis coach (usually 5000 lbs) and thus force you to a diesel pusher. The DP will be rated to tow 10,000-20,000 lbs.
A car-hauler trailer large enough for both car & atv is probably 1200-1500 lbs all by itself, plus the atv and the car.
The diesel vs gas choice goes far beyond the engine.
A DP (diesel pusher) chassis has additional features and capabilities, plus the coach built on that chassis is most always better-equipped than the nicest gas model. Buyers willing to spend more to get a DP are also buyers looking for more amenities and and features and have the budget to pay for them. Much has been written here about the one vs the other and a SEARCH will turn up those discussions.