Meh... Tow dolly puts very little weight on the tongue. The motorhome weighs substantially more than the vehicle and dolly. The dolly has brakes and reportedly, the frame is re-enforced.
Will it put you over GCVWR much?
I am contemplating going about 2-300 lbs over the 3500 lbs rating on my receiver as well. I know in my case, the limiting factors are the weak frame extensions and the hitch itself.
If your tire blows out on the dolly, is being a few hundred pounds over the hitch rating really going to be the determining factor in a disaster?