Cheap way to isolate and charge gel cell- relay or diode ?

We have a 19 foot Hybrid. We will not do a lot of boondocking. i have a 100 amp Gel cell and a gel cell charger that keeps it properly topped off.

Meanwhile, the campers 12V power supply would damage the gell.

So i want to isolate the gel from the campers 12 volt system when I have shore power.

One option would be a 110V relay that cut out the gel whenever 110VAC is present

A cheaper simpler option would be a diode to prevent the camper power supply from putting voltage to the battery.

But i wonder the battery would charge properly if it also had communication with the rest of the DC system?

Is there an existing product in the RV tool box?


thanks