09 Dodge 2500 4X4 mileage and front axle question

I have a 2009 Ram 2500 quad cab 4X4 with the 5.7 liter and 5 spd auto trans, and we want to go on a trip this winter with our 32 ft 5th wheel camper, approx 12K lbs, and wonder if anyone knows what kind of mileage and power we can expect. We normally pull it with a 3500 dually cummins but it is only a 2wd, and weather may not cooperate. Also, do the front hubs unlock when the truck is in 4X2 mode? It has a manual shift transfer case and it looks like the axle and driveshaft still turns when the truck is in 2wd. Does it just disconnect the transfer case and the axle and driveshaft stay connected? Thanks in advance.

I think it only disconnects at the Tcase.

As for mileage, plan on "sucky". The HEMI can get 20mpg unloaded, but with a 12000 pound trailer I'd plan on less than 10. Use caution, sounds like you will be at or near the maximum towing capability.