We have a 36 foot fiver, and after our(my) last mishap are now effectively shut out of my favorite place. You will find many of the NF camp grounds were built 30-40 years ago when RVs were much smaller and simply cannot accomodate large modern RVs. Heck, one loop of the state park were in now the large RVs have to be really careful to avoid trees.
comfort is really a matter of how well the living space is arranged, not how big it is. As a general rule I think you will find that for every foot over about 30' you will loose available camping sites. At 40' you pool of available sites will be drastically reduced to newer resort type parks.