You should make sure that both batteries get a maintenance charge - ideally you have a decent multi stage charger on your EHU already for the leisure battery so just leave it hooked up - starter battery you can either fit something like the AMT-12 to trickle it from the maintained leisure battery or you can plug in a small maintenance charger from CTEK/NoCo/Victron inside the van (given you will have EHU on) and connect it to the dash 12v socket by the gear (which on most vans is connected to the starter - but not later T6.1)
Don't do tick over, diesel engines (and I'm assuming you are) do not like running unloaded at all and it can cause soot and bore varnishing. Much better to just take it for 20-30 minutes drive instead and get everything warmed up and moving. With our Caravelle when we are not roadtripping for a bit I often take it for a bit of drive around the weekly supermarket run for instance.
If you're on EHU and find damp being an issue you could use a dehumidifier. You might just need a few of the "rechargeable" silica gel damp bags in the cupboards that are at risk, remember to leave cupboards, drawers and especially the fridge ajar to let air circulate, a couple of towels or blankets you may already have in the van works a treat. I have 2 damp bags on the dash and 2 tucked into the rear kitchen pod (which gets cold and air doesn't circulate well) and check them whenever I take the van out.