Like Excella CM said, it varies.
Possible sources of parasitic drain, even with kill switch off (not necessarily a complete list):
1 - Solar panel's charge controller. The panel may not be working because you're parked in the dark, but the charge controller is still trying to draw power to control the nonexistent solar charge. If the solar panel bypasses the kill switch, so does the controller.
2 - CO/smoke detector, if it's wired to the trailer batteries and not on its own internal battery.
3 - LPG detector, ditto.
4 - Inverter. Might be wired in parallel to the kill switch like the solar panel. If so, then any 110v appliance that uses power even when switched off (like a television or anything with a built-in clock, like a microwave) could be drawing a small amount of power through the inverter, unless the inverter is ALSO switched off.
5 - Propane appliances, if they have a piezoelectric ignition rather than a pilot light. You don't want propane appliances to flame out and gas you to death just because you shut off the electricity, so they'd also bypass the kill switch.
Even if it's not one of those, the batteries could be on their last legs anyway. If the batteries were drained too far before recharging, that would shorten battery life. The more often it happens, the more the battery life is shortened. It's not inconceivable that a nominally five-year battery could be repeatedly over-drained to become a three-year battery.
Disconnecting the battery cable from the negative terminals will definitely prevent parasitic losses, but only do that once the batteries have been fully recharged. Storing them only partially charged hurts their longevity as well.
Engineering: Finding complex solutions to simple problems you didn't even know you had.