Couple questions:
1: Got water/electricity/sewage? If so, a destination trailer may be the best course... but you will have to plumb everything due to no tanks. Those type of trailers are made to not be permanent structures, which help with property taxes.
2: If going for a trailer or fifth wheel in Washington state, I'd look for a four seasons model. They are more expensive, but well insulated and have tank heaters. Also make sure the rig supports full-timing. I know a number of TTs are "for occassional use only" which means they are designed for at best weekend use when it comes to construction.
3: Don't forget skirting. Skirting a TT or 5W will help keep wind from blowing underneath, add an insulating layer (although mice/rats can be an issue there), and also add some storage space. It doesn't hurt to toss a light under there as well for heating reasons.