A fully charged FLA battery won't freeze AT LEAST until -40 (or colder!). If your batteries are freezing and cracking, they are not fully charged!
It should be noted that batteries have self discharges that will kill a batter in a matter of months of it is not supplimentaly charged mean time. In addition, most RV aux batteries ae controlled through a relay, which has a considerble parasytic draw on the auz batteries. (I believe we have talked about it previously in this and other threads) The OEM one in RT agiles draw about .5 amps, enough to kill the aux battery in a matter of a week or so.
The simple solution (save installing a isolator relay that draws little or no current) is to simply install a postiv off switch on the battery negative, and whenever you park for longer than a day or so, simply turn off the aux battery. Turning the battery "off" from the control panel does not disconnect the battery entirely and indeed it will continue to discharge.
Finally, one should learn to check battery condition as a regular thing, especially in cold weather. Simply checking battery voltage is not enough, checking specific gravity in a flooded cell is essential. (one advqntqge of flooded cells over AGMs. )
Icarus