You pay through the nose for specific Motorhome tyres, most especially those made by Michelin. They do claim that they have a greater resistance to UV rays that can knacker a tyre which is parked up for long periods of time, but trying to prove that would be nigh on impossible!
Personally I run 'Millie' on Kumho van tyres and am more than satisfied with my choice.
As our van quite often go off road onto grass, one of the most important things to look for is a decent tread that will give grip.
Phil