Trying to decide the right size MH for full timer

Some, and perhaps most parks have at least a couple long sites. If you are full time, you should be able to plan your arrival to maximize your chance of getting a good site.

Second, there are parks (such as Zion) that actually measure your rig, but mostly they don't. You can usually fit a 40' coach in a 35' site.

Lastly, length only matters for the few minutes when you are parking or leaving (assuming it fits in the spot). Once you are moving (or parked), it makes no difference.