Another issue will be water. There are also regulations regarding wells. You usually can put in a well for drinking water but "agricultural use" such as irrigation of a crop or livestock may require water rights.
Lastly, you may not be allowed to use a Travel Trailer as a permanent residence unless it complies with various codes. Usually these cover the electrical systems.
Here is the site that lists all City and County codes in WA State.
Find the county and do a search for Travel Trailers, RV's, etc. This will give you the "straight poop".