My 12 volt dc heating blanket works well. It draws 6 amps and does eventually start to cycle. I wish I could find a mattress pad as that would be even better.
If you wish to power a heater during "solar noon" you may run a 300 watt heater for about 90 minutes before and 90 minutes after. However, that may mean there is not enough sunlight left to return the AGM to fully charged.
If you had a firefly battery bank fully recharging needs not be done every day--but should be done once per month (or more often).
Has anyone simply connected their panels directly to a dc heater?