My 20ft definitey has much smaller wheel wells than my 28ft
20ft originally had 16.5" wheels (29.5" diameter including tires), now changed to 16". I have 225/75r16 tires installed and they fit the wheel wells (29.3" diameter) but I tired 235/85r16 tires (31.7" diameter) and even though they are still 16" they rubbed the rear side of the wheel well (when under load). I have a set of 19.5" wheels (33.6" diameter with tires) on the 28ft and they would not even fit within the wheel well of the 20ft so couldn't be fitted.
Maybe this is specific to my 20ft as it has a new frame (supposed using the original as a pattern) and also the wheel sits closer to the rear side of the well.