Just to recap: you want to use your home WiFi as a WAN source, when the campervan is parked stationary, and use mobile data, when you are on the road?
Could you check if a simple failover configuration would not be enough?
Basically, configure your device as a wireless client. Login to router's WebUI, navigate to Network -> Failover. Drag wireless client interface above mobile one to provide it with a higher metric value.
Additionally, edit settings for each WAN interface in the same Failover page and add the following:
Your home WiFi should become the preferred WAN interface each time it is available and all traffic should be routed through it.