In addition to limited camping at NPs I have found that fuel at the truck stops is more expensive that at a nearby "gas" stations. Any length over 30 feet will be a problem getting into and out of a gas station.
Also think about how you are going to use the 5er. On our (3 adults) 6 week trip from TN to UT and CA we spend very little time (only to sleep) in our 5er. During the day we are out sightseeing so we do not need a very large 5er. We also like to cook outside on a grill or propane camp stove.
Even with a 30 footer you need to make reservations at NPs 6 months in advance. For example at Grand Canyon at the NP Mather Campground they only have about 10 sites (out of several 100) that are for 30 foot or longer and only 1 or 2 that will take a 40 footer.
