Take a look at Toy Haulers. They have ramps to get quads and motorcycles in but there is typically a door from the garage to the living area.
This would allow a scooter to just drive right in or at least it's up a gradual incline. If you really need it, I'm sure you could fabricate an extension to make it even shallower slope.
I think a lot of it not a lot of handicapped people RV'ing, so it's a small market. I'm sure a good welding shop could fabricate something that swings out from under the trailer and forms a lift in front of the door.