Joe is right on about the legal aspects. Another thing to consider is capacity of the tow vehicle and also the trailer frame. Most 5th wheels are not designed to tow anything behind them Adding a hitch is only one concern. The frames on most all of them would need to be beefed up all the way up to the axles. The tongue wt also effects the balance.
I know several folks that pull large heavy 5th wheels and take two big bikes along with them. They are using a HDT or a larger MDT truck with a custom bed on the back of the truck to haul two bikes. This way the bikes are loaded and hauled safely and legally. No effect on the trailer and you can chose any trailer you like.
Have a look at this forum and see what they are doing.
See Jack Mayers web site here http://jackdanmayer.com/registration...ouble%20Towing
He has some really good stuff on this.
Not for everyone, just another option though.