Here's the skinny on replacing your group 24 batteries with group 27s:
*remove battery cables (negative first)
*pull the batteries and remove the threaded rod (should be a nut welded to the bottom of the battery box.
the plastic tray is unnecessary and you can trash it.
drill out the 3 rivets holding the tray to the forward edge of the box and remove the cover.
remove about 3/8-1/2" from half
of the forward tray lip with an angle grinder (you may find it easier if you remove your LP tanks)
file, prime and paint the cut edge
place one new Group 27 in thru the new slot and slide it to the opposite side of the box
replace the threaded rod
place the second Group 27 into the box and secure the batteries with the wing nut
*re-attach cables (positive first)
re-rivet the lid and replace LP tanks and cover
With respect to adding a third or fourth battery inside the trailer, it is more desirable to place ALL batteries in the same environment. This will keep all of the batteries at the same ambient temperature, which is necessary for even charging, even if your charging device does not have temperature compensation on the charge voltage.
And yes, only place AGM batteries (and I only use Lifeline AGM
batteries for this purpose) inside the trailer unless you use an air tight, vented enclosure.