heating problem

[btinthedesert]

I have a 04 2500 durmax with the edge I live in the southwest (hot country) I pull a 5th wheel 30' toy hauler total about 12,000 lbs. While climbing hills and the edge in tow the heat climbs very quickly if it is a long hill the heat will climb to 240 and this will also cause the tranny to get hot. This is the first time this has happened, I also have been getting very poor fuel milage, example on flat ground at 72 mph pulling loaded trailer 6.6 mpg with the edge in stock as it will not run in the tow selection without the engine hiccuping any suggestions?????