Thanks Garry. I'll look at my extensions. I'm sure my RV is rated to pull more than the 3500 lbs that the hitch is rated for, but I may strengthen up the extensions a bit more if they look weak.
I'm thinking that the majority of the stress on a hitch would be during braking since the motor won't accelerate the rig with as much torque as the brakes will apply when stopping. Thus, the brakes on my dolly should help with that.
I'm not out to put the entire United States at risk, but I don't think 20% over is that big of a deal.