I turn the them both on the night before we leave home while the PC is still plugged into the house.
Tom - why do you find it necessary to turn the hot water heater on the night before? We usually turn ours on when we get to our campsite and then turn it off before we leave the campground. If we do forget to turn on the water heater it only takes about 10 - 15 minutes to heat up the water.
We also never use the propane on the water heater unless we do not have electric.
Just a reminder - never turn on your hot water heater if you do not have water in the hot water tank.