I have been getting away with this for years now with the 5er, no problems noticed. (yet?) Six batts up front (two not showing off to the right), no vented trays.
I do have the door up when charging from solar or from the generator and using the chargers (don't use converter for that) Gassing is strongest when near the end of the recharge after batts reach 14+ volts.
On shore power, 12v on converter, batts charged and floating, no gassing really, but any there is gets a way out using that cable hatch I put in the door, plus the side doors to the cargo bay aren't that tight.
I have the inverter etc in the same area and they have not been eaten by fumes either. Have not blown up the trailer from sparks.
Can't say it is safe and everyone should do this of course, but so far so good here!
