Its hard to say because the furnaces are not all the same in their electrical consumption. 6 or 8 amps is often given as a number but mine is rated at about 3 amps. Also they cycle and are generally not running continuously.
Here's a few tips:
Blankets are great, use plenty and set the heat as far down from 70 degrees as you're comfortable with.
Always be sure you start with fully charged batteries. Don't start a parked RV and assume they're still charged from last time, or that the drive will charge them up.
Read through your furnace instructions/spec and try to determine how much power it uses. Learn you battery amp hour capacity and how to figure run time for various loads.
Convert to LED lighting if practical or necessary.
Oh and the question about plugging in at the pedestal in another site... Prearrange it in the office-request permission-pay for it. Don't do it without discussion with management.