The heater should work fine when not plugged in. The fan will use a fair amount of power, but somewhat less than an electric fridge, in my experience.
To use the furnace you should just need to make sure the propane valve is open, then set the thermostat to heat and set it below the current inside temperature. Within 30 seconds or so the furnace fan should come on; it will try to ignite a minute or so later. If the propane valve has been closed it may not ignite on the first try. If you don't have heat in a few minutes, turn the thermostat off, let the furnace turn off, then try again.
There's a small window into the burner chamber that will let you see the flame, but it's hard to find with the grille installed.
If the furnace hasn't been run in a while you may notice a slight burning smell as accumulated dust burns off. That'll go away after it's run for a while.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.