NEED MORE INFO - as InterVec made their Class B's with Chevy, Dodge and Ford vans. They also used 3/4 ton and 1 ton chassis. Which combo do you have???
However, just by googling 1992 Chev, Dodge and Ford 1-ton tire sizes I find that the tire sizes ranged from LT 225/75 to LT245/75 sizes. Some were on 15" wheels and others on 16" wheels.
So, I'm guessing that someone prior to you put narrower tires on your RV. Also, as SteveJ suggested, look at the sticker on the driver's door that lists the "tire size" and recommended "air Pressure". That will tell you the correct tire size that your vehicle came with from the factory.
Will a larger tire help with wandering??? Maybe, but most likely No.
But I do think that your tires are probably narrower/smaller that original.
Now: 1998 Coachmen Starflyte FRB 21' Class B+
Formerly: 1996 Coachmen M19-RD GMC 19' Class B