Tire Pressure Questions on my Michelin XRV 255/80R22.5

You are figuring correctly. Each axle has two ends, so you divide the axle weight by two (and maybe add a little bit of a fudge factor since it's unlikely that the weight is perfectly balanced left to right). Something like 80 psi for the fronts and 85-90 psi for the rears would seem about right.

The tires can safely be used on vehicles a good bit heavier than your motorhome. That's not at all a bad situation to be in.