In WA state, you can't keep a LOADED weapon in a motor vehicle without a CWP..
But, if it's in a TRAVEL TRAILER, I don't know that anyone would ever find out, unless you are crossing a boarder into Canada or Mexico??
In that case, you can't even take a pistol into Canada at all... No matter where it is stored..
I carry my .357 mag pistol LOADED in TT or my truck, but I have a CWP as does my wife... :)
If I wanted to cross the boarder, I can't take it anywhere.
If I want to go to the East Coast, I'd just leave it in the TT and "don't ask, don't tell"... I know there are only a few states that will actually "acknowledge" my CWP in their state, so I'd need to read up on that and unload my pistol while in transit accordingly..
An UNLOADED weapon in any unaccessable part of the vehicle from the driver is usually OKAY, just as long as you can LEGALLY own a firearm..
I remember in high school in the late 70's, we had kids with those rifle gun racks in the back of their pickup trucks with RIFLES in them!! But that was a long time ago in a land far away and you can't do that anymore, anywhere... :)
Good luck!
Mitch