Does the camper have a battery that is charged via shore power?
Regardless... Replace all internal lights wih LEDs. They use a lot less energy. Could be easiliy fed by the truck battery.
Charging mobile devices can be done by a separate powerbank.
But a furnace (forced air type) will require a reliable electrical power source. The fan needs around 5A continuous power while running. Depending on battery size and how much it cycles, it can drain a battery in one day. Wouldn't want to experiece that with your truck battery parked in the outback...
Other electrical consumers may be fridge and water pump if equipped. Absorber (LPG powered) fridges do not use a lot of electricity while running on propane. Water pump usually only cycles briefly.
Check what equipment is in the camper and we can give better assessment of what you need to power it.