I haul an Polaris Sportsman while towing an 19’ Keystone with my Ram 1500 6.4’ bed. Here is what I found:
1. Toy hauler is really the way to go.
2. Triple check your payload. I don’t think you calculation includes the tongue weight or the WD hitch weight and that will push you way over your payload rating making this not even possible.
3. You can use atv lifters to put the atv more vertical and thus shorter length. However, the lift basically needs to be above the bed rails to make a difference. Most people just put the front wheels on a toolbox. There are off the shelf systems for this you can buy too.
4. If you are close to fitting. You can remove the wheels (or air) or strap the tailgate closed.
5. Double or triple towing isn’t really a valid option. It’s illegal behind a travel trailer (not 5th wheel) in almost every state. Where it is legal requires brakes on both trailers. It will also require some serious modifications to your trailer frame to support the added stress. And it will probably tow poorly with the lightened tongue weight.
6. Removing the tailgate or replacing it with one with a “V” cutout would work. With the tailgate removed, you need to be extra careful not to smash a rear window. There isn’t any room left to slow down. You give it gas to make it up the ramps and then immediately hit the brakes. Ramp storage is a pain with the tailgate removed.
7. The tall truck bed atv haulers are not recommended for half ton trucks. Most of them are for 8’ bed’s anyway. I don’t think you will be able to find one for an 5.5’ bed.