My “Millicent” is a very common school bus model. 1992 Blue Bird, 40 feet overall length, flat front, engine alongside the driver. Cummins 5.9, which I am told works well for fuel conversions -- provided it is new enough to have the inline injection pump as opposed to the rotary pump -- easy to see on left side of engine. My 1992 has the inline pump.
I just measured the width inside: about 90 inches at a BMW mirror height. Couple of inches less down low about a foot off the floor where there is a ledge that the seats mount to. Plenty of room for the cars, but not enough to get in and out without scratching stuff.
Millicent’s wheel “humps” are about 24“ wide and 9” tall. I suppose they could support some weight, but not concentrated on the top of the hump.
The humps begin about 11 feet from the rear and peak around 12 feet. This makes a gentler slope than my “beavertailing” of the last few feet of floor.
I think I have figured out what I would do if I were you. You have a camping trailer, so both race cars ARE going in the bus. I would build ramps all the way from the back door to near the 32 foot mark -- near the driver’s seat. Let these ramps continue sloping upward all the way to the front. They will wind up around three feet off the floor. Load by winch and unload by gravity (gently unwinding the winch, yes!). Avoid putting any structural members in the “center cavity” between the ramps under the cars so you can slip underneath and tie the cars from the underside. Gets tight at the back, but this may be the best type of access for tie-downs. The tracks should have “fences” for the tires to keep the cars going straight. Might want to use old street tires for transport.
You could incorporate some beavertailing at the back, but keep enough frame for the tow hitch, as I did.
As for the "tailgate", you may have noticed in my chronicles a sketch of a "bi-fold" tailgate. I have not installed the outer section as it is not needed with my pedal vehicles, but you might want to do it.
Storage for tools and parts below the ramps -- accessible when the cars are NOT in there.
This make any sense?