5th wheels at 40' or longer are considered to be pretty long. But you're still not likely to have trouble finding a site as long as you plan ahead. Spur-of-the-moment stops are more problematical with longer trailers. Older campground and quite a few Forest Service campgrounds tend to have smaller sites with more cramped roads. A 33' 5th wheel will probably fit in most campgrounds, add 7-10 feet and the story changes dramatically.
We have a 40' toyhauler and have never not been able to find a spot for the night. Sometimes it takes a couple calls but we've never had to spend the night at a Walmart or truck stop because we couldn't find a space to fit into.