You can read all the specs what a gas MH is 'rated' for until your eyes cross but when it comes time to head out RV'ing, vacation, etc if your towing a heavy toad the trip will most certainly not be fun for the driver.
I tow a Tracker under 3,000 lbs with a E450. On the straight away you can't even tell she's there. Start going up even the lowest grades and you know it's back there.
Most buy the lightest toad they can to pull behind a "gas" MH.
Unless you like hearing your trans screaming on the steep grades as you try to maintain the speed limit which you most likely will not as it floats out half way up.
A DP MH is the way to go with a heavier toad. Just saying.