Length is the measurement from the front of the rig to the back of the rig. Not what the manufacturer says.
If you have a tray on the back bumper of a 5er or TT, measure to the outside of that tray. 5ers it is to the front of the rig, not the hitch point.
Having said that, if you are close, check with them and explain.
Length has a completely different meaning in National Parks, federal parks and state parks. Some places such as Yosemite, a length restriction might be because of the turns in the road to get to the site, or obstruction on either side of the site which limit your ability to turn the tow vehicle to get the rig into the site. Frequently you will see lengths quoted as longer motorhomes allowed than trailers, for that reason.