Tire Selection limited by size?

Check again. I think your profile will be 2-twenty-5/75R16, and they are very common on Class C chassis. You'll probably end up with Load Range "E" tires even if your chassis first came with "D"", nothing wrong with that.

Some of us have gone from 225/75 to 215/85. The 215 has the same weight ratings as the 225 but it's slightly taller and slightly narrower. If you want to increase tire size, have a qualified tire shop look into 235/85. The 235 would be taller still, so clearance could be an issue. But if it's wider in 85 series than the 225/75 is, then you need to verify that the "Dual Spacing" is adequate, and that's determined by your wheels. IN NO CASE can you have less than minimum required spacing. If the dual sidewalls don't have room to flex without chafinc (kissing each other) you'll have tire failure in VERY short order.

Are you sure that is the original tire size? You can check the placard on the door jam to verify that it's the original size.

Here is a link to someone that has done that.

Bigger tires