Yup the length is supposed to be from the bumper to the hitch ball. Typically this makes the box about 3 1/2 to 4 feet less.
However, few campgrounds are stickler for such details. As suggested measure and call.
Our TT is about 27 feet bumper to hitch and we have never had a space problem but we have had very tight quarters, being wedged betweean motor coaches with slides and some very difficult to back into tree lined spots.
Some of the older campgrounds were designed around the then jumbo 18 footers, so space is at a premium. When in doubt we call.