The 3 foot variation between floor size and total length is always up in the air depending on who you ask. Private sellers as well as dealers routinely list it both ways. After just shopping for a new unit I'd say it's about 50/50. If anyone is shopping and must know for some reason (to fit in a garage for example) you have to just look out for yourself and bring a tape measure.
As time moves on the number in the model# seems to have less and less to do with anything. If yours measures 31 feet bumper to ball I'd forgot anything about 26'. It's either a 28' or 31'. My last unit was those exact measurements. I'd usually just call it a 30 foot TT for reservations. If a search turned up no spots at a particular campground I'd change it 29' and try again :)