First, welcome to iRV2.com.
We're glad to have you join us.
I and many others here can relate to your situation - in my case, I have a BMW K1200GT that I would like to take along but haven't come up with a method that I consider acceptable.
I would suggest that there are several factors that need to be considered.
1. If the manufacturer doesn't recommend it, it may be because the 5th wheel frame hasn't been designed to take the static and dynamic towing forces of the bike and trailer. The 5th wheel frame members may be too small or fragile (i.e., insufficient wall thickness of rectangular tubing) to accept a trailer hitch or receiver.
2. Can you tow a 37' 5th wheel, the motorcycle trailer and the motorcycle within the ratings of your tow vehicle and 5th wheel hitch?
3. Towing double is illegal in many states. Check the laws of the states where you'll be traveling - in some states, towing double may be legal, but special driver license endorsements are required. There are also legal limits on total combined length - will your rig satisfy these?
Racks are a completely different matter. If you'll use the Find function on the toolbar above to search for "motorcycle carrier", you'll find lots of reading material.
Since you haven't bought the RV yet, why not consider a toyhauler?