I bought a new TT in Dec 2014 and my first stop was to get the Truma continuous HW heater installed. I then left on a 144 day trip to Alaska and back.
There were a couple of nights where the temps got down into the mid 20's. (I left the cold water run at a trickle to prevent a frozen hose.) No problem with the Truma. You have to adjust the amount of cold water you add to the HW. I didn't have any problems. Cold water temps were probably coming in about 40 degrees.
As for propane consumption, it only uses propane when you are using HW. When not demanding HW, NO propane is being used. With the 6 or 10 gal tanks, unless you turn them off, they will monitor the temperature in the tank and turn the propane on to keep the tank at the set level.
I traveled by myself and could go two nights without a sewer connection before my gray water tank would get full. I stayed at a couple of campgrounds for 7 to 10 days. I would use the full hook-ups, open the gray water valve and leave it open. Maybe a day or two before moving on, I'd close the gray valve so I'd have something to flush the hose after dumping the black water tank.