Welcome.
I wouldn’t worry too much about the previous owner. All campers share the same couple of brands of appliances and they aren’t too complicated.
The water heater is likely drained and bypassed. First make sure the drain plug is installed. Look under the outside cover. Then you need to make sure it’s not bypassed. Do a Google image search for rv water heater bypass diagram” to see how to do it. There are a couple slightly different setups. After it’s in normal mode, run a hot water faucet until all of the air comes out.
The first thing the furnace should do is run the fan when the thermostat calls for heat. If it doesn’t even do that, start looking for blown 12v fuses.