Go right to the beginning, that is the batteries. Do you have 12+ volts.
You didn't say how old your camper is, so if the batteries are good, look for either a very big fuse (about twice the size of the normal automotive fuse) or a circuit breaker. Is there continuity there.
Does the board on your furnace have a smell of burning plastic or any signs of a short.
Does the fridge work on 120V.
Has the fridge gone into lockout. This can happen for several reasons, check your owners manual for the reset procedure.
If none of these help, you may have fried your board and may have to get it tested to confirm.