We've got a couple of smaller oil filled that helps in our rig.
One warning that we've learned here at the park, though: In really
cold weather, don't rely on the electric. The reason is that your black/gray tanks are heated by your propane system. Now, if you turn on the electric, it does ok in the cabin, but it also causes the propane heater thermostat not to switch on. And that, in turn, results in black/gray tanks that become every colder, eventually freezing.
Bottom line: If nighttime temps are bitterly cold, and if daytime temps don't even come out of freezing, it's best to stick with the propane. If, however, you do use the electric for some residual heat, then locate the thing as far from the propane system thermostat as you can.