Maintaining RV batteries

We have a new to us '94 Fleetwood Southwind with a GM 454.

The RV part has two 6-volt deep cycle batteries wired in series and a regular 12v battery for the engine.

We bought it last year and we kept it plugged in, using the RV plug you would use when you go to a campsite, however when we took it to have a few minor repairs done, it had to be jump started.

The mechanic said we fried all three batteries. He said we couldn't keep it plugged in without adding water to the 6v batteries. I didn't think you had to add water to batteries anymore. I didn't talk to the mechanic, my wife did. And she was gets flustered, easily.

I don't know much about RV batteries, but can I just buy a trickle charger/maintainer for the 6v batteries and a separate trickle charger for the engine battery?

If I have to add water, I can do that.

I am new to RV's, so I want to do it the right way.

Thanks!

Mike