Does anyone actually run a motorhome setup from a narrowboat battery bank when shore power isn't available?

Bit of an odd one, but bear with me. I've got a 200Ah lithium setup on my narrowboat (Fogstar Drift cells, Victron SmartShunt) that I'm fairly happy with, and I'm wondering whether it makes any sense to tie a motorhome into that when we're moored up rather than running a separate leisure battery bank in the van.

The motorhome is a fairly modest affair — 2019 Fiat Ducato base, single 120Ah AGM at the moment. Mostly used for weekend trips but occasionally parked up near the boat at our home mooring. The idea would be something like a DC-DC charger keeping the van battery topped from the boat bank, or even just a decent Anderson plug run between the two.

Has anyone actually done something like this, or is it just asking for trouble mixing the two systems? Worried about voltage drop over a reasonable cable run (could be 8–10 metres between the two), and whether it's worth the faff versus just fitting a second AGM or upgrading to a small lithium in the van itself.

What sort of cable gauge would you need for that kind of run, and would a Victron Orion-Tr Smart be overkill for something this occasional?

@AshSeeker this is basically what I do in reverse — I've got a shepherd's hut and a narrowboat, and I've cross-fed between them during emergencies more than once.

The main thing to sort is your transfer cable and whether your motorhome's charger will play nicely receiving DC from an external bank rather than going through a proper shore power hookup. Most people run it via a Victron Orion DC-DC to keep things clean and avoid any BMS conflicts between two lithium systems.

Your SmartShunt will give you decent visibility on what you're pulling out. Main gotcha is voltage sag under load — if the motorhome kicks the compressor fridge and something else simultaneously, you'll see it drop fast.

What's the motorhome's battery setup? That matters quite a bit for how you'd configure it.

Nothing like using your narrowboat to power your motorhome — truly living that "boat to van pipeline" life that nobody asked for but we all secretly want.

My static caravan setup taught me that cross-feeding between two battery banks is brilliant until both decide to die simultaneously at 11pm on a bank holiday, so make sure your Victron MPPT is doing the heavy lifting on replenishment or you'll be doing the narrowboat equivalent of push-starting a car.