The best thing to do is to turn on anything that you would want to run while your rig is parked, and measure it with an amp-probe. ?That will give you the total amperage draw, and you can then size your feed accordingly, based on the amperage plus line loss if the cord is long.
When at home, I use an adapter on the 30 amp cord so I can plug it in to a wall outlet in my basement for that purpose. ?I can run anything except the AC unit; the batteries stay charged, and I have use of the lights if I want to work on anything. ?I can turn on the refrigerator so it's cold for departure. ?I'm really not too restricted. Most of the dometic refrigerators draw maybe 2.5-3 amps. ?Information in this regard is on Dometic's website
if you want to look up your model in particular.
With regard to portable oil filled heaters, for the sake of discussion, I happen to have an oil fired radiator at the house that has selectable wattage (600, 900, or 1500 watts). ?To figure out the amperage from the known wattage, divide the wattage by the voltage. ?600 watts draws 5 amps (600/120), 900 watts draws 7.5 amps (900/120), and 1500 watts draws 12.5 amps (1500/120). So, with a 15 amp line, you can run the heater, but not much else. ?It would be a better situation to just run it on the lowest setting if you must use it, but realize that it may never bring the rig up to temperature and may run all the time, even with the thermostat. That can't be good for your electric bill, and the use of a portable heater bring other safety considerations to mind, so be careful if you use one. As a comparison, some of the hair dryers will consume 1500 watts. A ceramic heater or other appliance should have their amperage on a plate or label attached to the unit. If you don't have the means to use an amp-probe, just add up the wattages of the refrigerator, furnace, lights, etc., and divide by 120 (as in 120 volts) to get you close.
I don't use a heater in the motorhome when it's just parked, no need to, as it's winterized. ?No use attracting field mice. ?If I want to work in it for a few hours, I just open the gas valve and turn on the thermostat.