Bongo Fury

What I would do is have a manual changeover switch so you could connect solar panel to the SB while the van is laid up at home to prevent the alarm draining it, & then flip it to the LB for when you're camping where there is no hookup. Put a sticker somewhere prominent to remind you to set the switch to the appropriate position when you park it up.

The other option which I tried (& failed) is to connect the alarm to the LB. This will cause all sorts of issues if it uses voltage drop from the interior lights to trigger it unless you reconnect those to the LB too. A decent alarm should not flatten the battery to the point where it will not start. The Clifford alarm I used to have could be left alone for several months with no ill effect.

I supplied a customer with a dual battery solar regulator - it's just a solar regulator/charger with outputs for two separate batteries. It charges both, and you can tell it which battery has priority as a matter of % - i.e. LB gets 90% priority and if the SB needs a charge, it gets the other 10% (otherwise 100% goes to the LB). This can swing either way.

Works great but he had problems with it setting off his car alarm as the voltage changes in the SB were triggering the alarm. We could not find a solution.

On my own Bongo, my solar is hooked up to my LB, and then I have a two way switch which toggles the solar battery charging from LB to SB. I've never needed to use it, but I know if I need it I can flick the switch and divert the charging to the SB.

To be honest, unless you have a serious electrical problem, in a vehicle that has a leisure battery, the chances of running your starter battery down are slim to remote. It's not like a normal car, all the things that usually flatten a SB are now running off your LB. That's what it's there for.