Welcome to the Motorhome Club Rob.
There should be an OEM sticker on the dash or door frame area indicating GVWR etc. that also should show axle capacities and tire size.
I believe it is illegal to not have this sticker, so it should still be there somewhere.
Before you purchase tires, you might also load your unit with the max. amount your are going to travel with, and have each wheel position weighed. This will give you a good idea of what you are going to need for tires, and will also indicate what items that you might have to leave at home on future trips.