Would they be able to tell anything from rims, besides width? Maybe also bring photos of the rig with rims on, and measurements of clearance (height and width).
I you can fill some or all tanks before measuring, and pile in 500-1000lbs of stuff, that would help, too. Still won't give you steering clearance info.
When I recently had to mock up camping weight in my rig for a shop, I put all my patio plants from my S&B in. Big pots of dirt and sand are heavy! And it was easier than moving the camping gear in.
If all they can do is guess, ask the tire shop guys if they would be willing to put on some old tires from their recycle pile, and you or they drive around on them, to see if they rub anywhere.
I just make this stuff up; more than likely somebody who actually knows will chime in here, in the morning.