ItsyRV said it best.
We fulltimed for 15 years in a 40' motorhome. Never had any trouble finding spots. Yes, there are some campgrounds that cannot accommodate the larger rigs. Be glad they tell you that up front. Too many times I've heard, "We just had a 40-er in here last week" only to find out we couldn't fit.
Typically older campgrounds and state parks cannot always accommodate, but you just have to do your research. Once you find a potential park, look it up on Google Maps and look at the satellite view. That will help.
Dale