The answer to your question depends a lot of the rv, I will describe some differences and what I'd do
First, I just read a post from someone who has a Magnetek converter.. I would not council him to leave his rig plugged in all the time till he replaces said box (He is planing on replacing it) with a real 3-stage converter charger such as the Progressive Dynamics Intella-Power with Charge Wizard (mine is a 9180) The reason for this is that the Magnetek is not regulated and will boil batteries dry in very little time if the rig is not in use... It also won't run electronics w/o the batteries in place. (MY P.D. unit does a great job of running a ham radio station w/o battery assistance if I screw up and hit the STORE switch)
On the other hand, given I have a "Best" class converter (The PD is not the only one wihch uses the charge wizard, some even come with it already "Built in") I leave mine plugged as much as possible (I spend 2-3 weeks boondocking each year, And several days driving around the country, during these times I'm not plugged in though I may be generating power with the ONAN)
In the first year I added water once, at the 1 year point. Was not yet urgent but they could use a bit of topping off
Batteries still give "like new" performance (They are around 19 months old now)