An electric heater will likely NOT keep the water lines from freezing. Many RVs blow hot air under the RV into the bays when using the propane furnace. Too many run pipes under the RV where that heat will not get to.
We have always blown out the lines and either drained the tanks or left the tanks with lots of head space to allow for expansion in case of freezing temps. Simply draining will leave water in very small pipes going around corners, a great place to freeze and crack. I would also drain the hot water tank. GOOD LUCK!
