In looking at your 54 posts I do not have a feeling of how old you are and how long you plan on RVing. In that small a rig you do not need a diesel. I realize that 30+/- is bigger than what you are used to but staying in public CGs like NP and SP is not the reason to go that small...especially when you said This is exactly our plan, FHU with occasional boondocking. Out West we boondock 5 out of 7 days/week in our 42'. Do not compare the length of a MH and car to that of a 5er and truck as the car can be parked 90 degrees to the MH taking up 8', not 18' of a truck. Yes, some sites will let you park a truck 90 degrees.
Tank size is VERY important for boondocking...$6/night in most USFS CGs with the old farts pass which now costs $80 one time.
Here are some of my posts on going from 36 to 40 to 42'.
Here is how we boondock in USFS CGs without reservations.