Is it smart to Register a 5th wheel in Maine

You'd have to look at your state's laws. I live in Maryland, so as long as you have one trailer registered in Maryland, any additional trailers are considered part of a "fleet" and can technically be registered in Maine if you are engaged in interstate commerce. Sell a hotdog to a friend over state lines and its interstate commerce!

But the company I used (registered my boat trailer in Maine) wouldn't register RVs. I paid $99 for 5 years of tags in Maine. Maryland wanted $180 for an inspection (not required in Maine and the trailer didn't have brakes anyway), plus $180/2 years of registration. No thanks...