I think most "seasoned" RV.net-ers have learned to use the actual bumper-to-bumper length of their camper, if that be TT, 5er, MH, or pop-up. Hybrid owners almost almost always use two figures in their description. Bumper to bumper and bunk end to bunk end expanded. I don't think too many refer to the actual floor space inside the camper. That's just too confusing.
You definitely cannot use the dealer description either. Every manufacturer uses a different figure. All you can assume is a Trex-175 is going to be shorter than the Trex-275.