We do three months out West without reservations and do not have problems finding sites for 42' in public CGs. It takes some advance planning and you do not show up Friday hoping for sites big enough. I like this site for finding
Public CGs. Just because a USFS CG says 30' max, it does not mean there are not any sites bigger than that. Other than NP if you arrive Monday there are plenty of site open.
When I search trip and West I get 2000 posts so searching under roads and routes, or RV Parks/NP, or Public Lands, Boondockiong would help.