We are 40-feet and we DO have problems finding sites. But that's because of the type of camping we like to do. We primarily enjoy remote camping and public parks such as state parks, national forest, BLM, boondocking. Our 40-foot size is often an issue esp. in forest sites in the mountains (such as CO in the summer) and for getting into boondocking sites. I have to do some detailed planning to make sure we fit.
In private parks you won't have any issues, but if you enjoy more "remote" camping smaller is better. The type of camping you like to do is key. In retrospect we wish we'd gotten a smaller rig, but we may be unusual in that respect. Our next rig will undoubtedly be a smaller one so we can get into more remote spots.
12 paws, 40 feet and the open road