2-3 pounds per hour depending on size of A/C and duty cycle, thus about 13-19 hours off a 40 pound tank, with some reserve, unless your "10" is water capacity, then it is really an 8 gallon, or 32 pound tank, and the reserve is gone. If lives are at stake, assume the higher level of consumption.
At max load, the genset is going to use about 5-6 pounds of fuel per hour. Gensets and A/Cs in motorhomes are not sized to make running the A/C a maximum load. You would never get the A/C started if that were the case. Practice is usually to size so that the running A/C is about 30-50% load for the genset.
Diesel genset on a Sprinter, if you can find it, you still have a fuel supply problem, though it is probably easier to make sure you park with a full tank. Because MPG is high, Sprinter tanks are much smaller than the 35-60 gallons on Ford and Chevy Cs, and as with those, the genset doesn't get to use the whole tank. Figure about 20 hours max for a diesel genset pulling from a full Sprinter tank.
Ford and Chevy B's will also have smaller tanks than those provided with the heavier motorhome chassis. The vans don't usually have the auxilary fuel tap, so how much of the tank the genset can use may depend on te RV manufacturer's installation choices.