I don't know of any device that will do what you want. It isn't simple. After the batteries reach float via solar charging you could produce AC power from an inverter to heat hot water, but unless you had a really big solar panel system, you would almost certainly be drawing down the batteries.
Best do it manually. I have a boating friend who does this. He replaced the 120V electric element in his water heater with a 240V one which drops the wattage from 1,500 to 375. He has 400 watts of panels, but even a 375 watt draw is more than his solar panel system normally can provide, so he has to watch it and turn the water heater off if it clouds up and make sure that there is enough sun to fully recharge his batteries after the water heats up.
The point is, he can only do it with reduced water heater wattage, full sun, large solar panel system and careful monitoring. Not something a controller can do easily automatically.
David