Sounds as though you have it figured out allready. The greatest concern is that they do not bubble and the best way to achieve this is by keeping the voltage down to no higher than 13.2 volts at 12 vdc nominal provided that *cold weather isn't a factor. 50 watts of PV should do nicely. Check the modules "working" voltage, no need for an 18 volt module when a 14 volt or below would do the trick. Any solid state PV charge controller that can be set to a high voltage of 13.2 at 12 vdc nominal will do. The old off/on relay type PV charge controllers have the potential of "sticking" with age, I wouldn't recommend that type for an un-maned system particularly. Mind you this is just for a storage maintenance charge. It would be a really good idea, I dare say necessary, to invest in hydro caps. 18 months is a long time without supervision. Unless of course these are sealed batteries. You may want to look into desulfators as well. It would be a good idea too, when they come out of a long storage like that to; top off the water and run them through an equalizing charge before putting them into service. Be sure all connection are clean and tight and that any exposed terminals are coated to resist corrosion as well be sure that all loads are removed from the batteries.
http://www.solarhaven.org/BatteryDesulfator.htmhttp://www.semarine.com/store/product.php?productid=315&cat=212&page=1*If cold weather is a factor it might be necessary to bump up the voltage a little, say 13.6 but definitely no higher than 14 because this is when bubbling starts and with no one there to replace water in the electrolyte well, you know the rest I am sure.
