Gell-cells and AGM batteries don't vent hydrogen (much). I think a fairly open container would work okay for both LP and batteries -- expanded metal mesh bottom, expanded metal mesh top. I have never noticed my propane tanks leaking. Not that propane tanks don't or can't leak, but seriously, if your tank was leaking, that's probably something you'd take care of right away. I don't think that storing the two items near each other is incompatible, even if you are using regular lead-acid batteries that do vent H2, as long as the storage area is ventilated and does not contain a likely source of ignition.
If you get into a wreck that is severe enough to damage the propane tanks, you're likely to have plenty of ignition going on, anyway, so that risk is always present.
OTOH... It's your rig, not mine!