Yes, you can use a maintenance free wet cell. If you use two batteries, make sure they are both the same size and age otherwise the charging and depletion might be off sync.
Detectors : Pull the fuse when you store it or install a toggle switch on the detectors if its running down your batts.
Simply unscrew the detector, pull it off the wall and you will see the two thin wires powering it. Spice the toggle switch into the wiring and install the switch next to the detector. Use it when you need to.
Another option is to install a knife switch onto the battery terminal to kill the power when you park it.