24ft enclosed trailer. Probably around 8K. Towed for five years with my 95 1500 3.5 geared 5.9 gas 4x4. Had to pull a couple grades out here in 2nd gear at 25mph. Had three tranny coolers total on that truck to keep things livable. I would still see close to 300. Once you got on the flats and could achieve torque converter lock up it would drop to 180. Only time that truck towed "ok" was on a dead flat with no, or a tailwind.

Current rig is a 01 2500 3.5 geared 5.9 CTD 4x4. Chip/injectors/built trans by me. Those same grades I pulled at 25mph are now 55-60. Set the cruise at 65-70 and GO.