Public parks are our first choice with our 40' MH - national parks, national forests, state parks, county parks, COE, city parks. We have traveled east of the Mississippi but most often are west of it. Our MH is listed as 40' and we have no qualms going to a campground that states 40'. We have found that the western states have much bigger sites than the eastern - except California and chances of finding a 40' size in a state park there are slim... but other public parks, yes.
We have volunteered for national and state parks. We know that when they state 40' that is the 'average' for the site size. We have never found a rule stating that you will be measured and if you're over 40' they won't let you in. This is definitely not the case. Where we volunteered we always told customers that if you're comfortable entering the campground and you can fit on a particular site, you're most welcomed to stay.
I will say that if we're not sure of the campground we'll unhook the towed vehicle outside of the campground and drive in to see if bringing our 40' into the campground is feasible (roads, turns, trees). We have found very few campgrounds in our 16 years of full-timing that we haven't fit and that includes the western national parks and forests. Also, we don't typically make reservations and we seem to find sites for us and sometimes the best ones in the campground!