Tire off rim- cannot get it seated

[Robin]

My coach has one funny valvestem on the rear outboard tire. I was
unable to get a pressure gauge or air chuck on it. I put a U-turn
adapter on it and inflated it a bit. Well, after sitting a couple of
weeks, I noticed the tire was flat (I'm assuming the adapter leaked).
Inside tire was low also. When I pulled her forward out of the
shrubbery and nearer my shop compressor, the outer tire unseated from
the rim. I tried putting a rope around it and cinching it down, but
no luck. My compressor will not put out enough air at sufficient
pressure to seat it. I am thinking I need to call a mobile truck tire
service (can they replace the valve stem at the same time?) Any
wisdom about getting it to seat before I do? I imagine taking it to
them will save me a few bucks if I can swing it.

John Churchill
Sanibel FL
"Large Marge"
1980 FC33