Agree w/ Old-Biscuit. I wouldn't worry about 1' or so wiggle room. That's usually just fine. We're a 40-foot RV, but our size is actually 41.8' measured end-to-end and we regularly stay in 40-foot sites all over the place.
Lengths vary a lot by campground. In some of the older parks (esp. public parks) the access roads are more trouble than the campsite itself. So you can run into problems with trees, tight turns and such. Not usually an issue unless you're really trying to "squeeze". We've put our RV into 35-foot sites in some places....it can be done, just depends on the park.