Our previous 5th wheels were 28, 34, and the current is 40. I've never had an issue getting a site. We prefer COE and state park campgrounds, but will stay at a nice private campground, especially for just an overnight.
I have looked at parks in New England states that had small sites, but they didn't have water or electricity either. They really don't like RV's so I accommodate them by not spending my money there.
We plan ahead and reserve early.
If you travel in the same direction more than once you will remember which parks and campgrounds you like and can accommodate you.
We haven't parked overnight at WalMart yet.
Get the RV you really want.
40-43 ft isn't that rare anymore.
edit add...you will have issues in National Parks, especially like Smokey MTN NAT Park. They are a few there but go fast.