It can be done but expect to weld on a lot of reinforcing and then you are likely to have a light pin weight which can negate the handling benefits of a 5th wheel. Keep in mind, the hitch ratings are assuming a trailer...hanging a bike off the back has a different dynamic loading.
Toy hauler is designed for the job and the weight distribution already accounts for the weight.
A swivel wheel trailer does a better job as at least 50% of the weight is on the trailer not on the back of your 5th wheel and you can still back up without issue.
If you are sticking to states that allow it, triple towing with a standard utility trailer is an option.
