Levi,
You have heard a lot of good info here. Here is one other thing to think about.
I'll be headed to Tri cities about that time. I've got a Ford dually and I could haul it back for you for diesel money for the return trip to Everett. I'll be doing a round trip from Federal Way to Yakima to Tri cities and back.
I have a 40 ft. 5th wheel myself, I also teach RV and trailer driving, so could also give you a bit of instruction.
I agree with above, less than a week to get the truck set up, hitch installed, and some practice in isn't much time.
If you have towed a trailer before, it isn't too bad to switch to a 5th wheel, but there are some differences. Have you camped in an RV before??? you don't state your RV ing experience in general.
And don't rely on either of the salesman to give you the best advice. Especially on what truck to buy. You didn't tell us which Montana you have or are getting, but you want to really make sure that you get a big enough truck. both will tell you a 250 will tow anything, etc. etc.
If you want more info, feel free to PM me here and we can talk. I'd hate for your first trip out to be a nail biter for either you or your wife, as this is a great lively hood.
Gary