Step 1, rip it all out 'cept the wiring
Step 2, buy a CTEK 7000 Battery Charger/power supply $170 on Ebay or 200 bucks for the 15 amp job.
Step 3, connect the CTEK to the 240 Volt and the 12 Volt output to the existing Camper wiring (it even has interchangeable leads to do so).
Totally do it yourself and totally safe plus the CTEK will work purely as a power supply off 240 Volt without even having the Battery in the Camper if needs be AND you have one of the worlds best Battery Chargers.