2012 Expedition EL w/HD tow package, longest travel trailer?

It's the 2001 keystone montana mountaineer 31-32' which means it's either the 305FKS, 310TBS, or 315RLS.
Dry weights range from 6160-6600. And hitch on 2 of them is over my 890lb max (980 lbs) and the 310TBS hitch is only 740, bit ways 6545 dry. I didn't ask which model it was since those weights just don't seem possible to me.