Hi Paul, you've stumbled onto a forum that's dedicated to all molded towables. A completely different build type than your 5th wheel. Ours are (typically) small and built like boats... no internal framework.
You're hoping for answers to your questions, and we may have some that can help, but I'm betting you will get the answers you seek at a couple of forums dedicated to your build type.
I respectfully suggest: www.iv2.com - Top Searches on the Web
and the Open Road forums at RV.net
Best of luck!