Had this exact problem with my shepherd's hut setup last winter. Took me weeks to track down the culprit, so worth sharing what I learned.
First things first—measure your actual drain. Disconnect the battery and put a multimeter between the negative terminal and cable. Should read virtually nothing (few milliamps at most). Anything over 50mA suggests a parasitic drain you need to find.
Common suspects in order of likelihood:
Habitation electrics — Caravan alarm systems, control panels, and chargers draw continuous current even when "off". I had a cheap leisure battery monitor that was pulling 80mA constantly. Replaced it with a Victron BMV-712 and sorted.
Fridge — If you've got a compressor fridge running from leisure, it's supposed to draw ~5A when active. But faulty thermostats can cause constant cycling. Check it's actually switching off.
Solar controller — Some cheaper MPPT units don't have proper low-power modes. Mine was drawing 40mA in darkness. Upgraded to a Victron MPPT and eliminated that entirely.
Wiring issues — Corroded connections create resistance, which generates heat and phantom drains. Check your battery terminals are clean and tight.
What size leisure battery are you running? And do you have solar, or purely relying on hookup/alternator charging? That context matters because a 100Ah battery losing 2% overnight is different from a 50Ah doing the same.
Also worth noting—cold weather significantly affects battery voltage, so if you're checking when cold, you might be seeing voltage sag rather than actual drain.
What's your setup in the van?