12 volt wiring on an old jayco caravan without a battery

Hi, I recently got myself a 89 26ft jayco with full ensuite which i removed from a permanent site in melbourne and towed back to W.A.
It has under the dinette seat a battery eliminator which the 12v pump is wired to.
It had a household type toilet in the ensuite which I removed and installed a dometic cassette type toilet, this requires 12v for flushing, I also removed the old gas hot water service that was rusted out and fitted a gas/electric suburban HWS which also requires 12v for gas only ignition. Now currently the 12v lighting inside the van runs off my dual battery while plugged in to the car. What I was thinking about doing is wiring the toilet, to the battery eliminator so we will be able to still use it when on 240v.
Can I wire the toilet, sink / shower water pump and hws igniter also to the caravan 12v power cable which feeds the interior lights so I can use all these when free camping without fitting a battery to the van? It would all run off my deep cycle battery in my car while parked up for the night. At most the water pump and a light would be on at any one time( changing to led globes). Would the original wiring handle this if I tapped into the power wire for the lights? Also if so would I need to install a switch to swap from battery eliminator to the 12v from my car? We wont be using the van too often to install a battery in the van. Thanks hope you guys can help.