It doesn't happen when you're not static, because you're probably recharging the engine
battery by driving more often. My experience was similar. If I drove the van every few
days, and didn't use the house batteries, I was fine. If I didn't drive it, and used the house
batteries, or just forgot and left them enabled, the engine battery would go dead after 3 or 4
days of just sitting in the driveway.
Is there any way you can stop and camp for a few days, without using the inverter, to take it
out of the equation, and see if your engine battery still drains?
The little light bulb may have been enough to draw down your engine battery over a day
or two, particularly if your engine battery has been "deaded" more than a few times and is now
weakened. Turning it off when you're camped, may solve your problem of dead engine battery.
If it still goes dead while you are camped, and not driving the vehicle for a few days, and there
aren't any draws on the engine battery at all, I'd still suspect the isolator, or that the house electrical
system has somehow been connected to the engine electrical system, possibly through mis-wiring of
your inverter system.
If you can't figure it out, you could prevent the engine battery from being drained by putting in a
disconnect switch of some sort to keep it isolated while you're camped and using the inverter.
You said you got an inverter to allow you to use 220V in the 110V system? Could it be miswired into
the engine battery? Could you do without it next time you camp, and just use your generator or
house battery to run things and see if the problem happens again?
I'm grasping at straws, 5500 kms away.