12 volt system

Hopeful that I have finally found a decent spark after a cowboy nightmare and should have things sorted this week.

During this ongoing sage I had a second battery fitted as a back up to the cranking battery, which was really struggling with an unidentifiable parasitic drain.

So looking for ideas as to how I might make use of this now its there.

Most switch panels I have seen have a combination of USB, 12v lighter socket and switches. The first two would be an obvious benefit and useful addition to the existing 240v EHU set-up, but given that everything else in the back of the van works directly from the ignition how are people making use of 12 volt switches ?

Additional lighting ? My thought was an alternative supply for the ICE so I could listen in the evening and turn it off without getting out of the van. (I'm past clambering across the seats).

Thanks in advance for any ideas and suggestions.

Parasitic drains can be a real pain.
The sunroof computer takes a bit BTW as do some radios, Pioneer is/was known for this.

Look in the custom section for my campervan electrics, it is a but convoluted though!!
I basically use the van 12V system for all 12V items, but have it on a float charge from my 24V domestic system, so any current gets drawn straight through the domestic system without the van battery noticing. I.e. it remains connected, but plays no part in supplying current when the current demands are within the range of my 15A buck float charger.

To do this you will need a dual boost/buck regulator from your 12V domestic battery, as the van battery needs to float at 13.2V exactly, but your domestic 12V will be around 12.5V tops.
E.g: https://www.ebay.co.uk/itm/202816254084
This will give you a 5A float charge so your domestic battery sacrifices itself first.

Note that if you do this the regulator is a four terminal device. That means it has an earth in and an earth out. It uses the earth out to regulate the current and stop it's transistors blowing. I.e. Nowhere in the van must you connect the earths together while you are float charging with this regulator because the regulator will blow in a few ms. So use another relay to isolate the domestic ground correctly - your sparky should understand.
I.e. you now need to think in terms of ground current paths, not just simple +ve voltages
It would have been easy if the current sense was in the +ve side, but it never is in any decent size switching regulator for some reason so we have to live with it!

I have remote control for my radio but it still needs 'ACC' ignition key position.