Glass mat batteries don't like or need the occasional equalizing charges from the inverter's charger that knock precipitant sulfur off wet cell plates, thus the necessity for telling the (I assume) Magnum what kind of batteries you have. I'm presuming it only refers to the house batteries, as they are charged first by the inverter off park or genset power. Upon reaching about 13.3 volts, the BIRD switches charging over to whatever's needed for the chassis batteries. I doubt that would include equalizing if you have AGM's configured on your panel. It may be necessary to check dipswitch positions on your solar controller, as I recall it needs to know the house battery type involved also.
I'm not sure why you didn't see Bulk, Absorb, or at least Float charging after programming for AGM's, but then I have all wet cells and there may be some particular reason that someone else here is familiar with on AGM's.
As to the water tank, there should be a water dump switch in your wet bay; hold it on for a couple seconds to make sure the valve opens, and you should hear the motor opening it and then see and hear water splashing from the tank underneath, perhaps on the other side of the coach. Be sure to close it again (hold the switch for a several seconds) when finished dumping.
There are a couple of methods to get a cup of bleach into the tank. One common one is: hook a hose to the city fill, add bleach into the hose, and attach the other end to faucet (without losing the bleach), and fill the tank. Run each faucet on the coach until you smell bleach. Let it set for at least several hours. Dump the tank. Of course use gloves, old clothes, eye protection, and ultimate caution... it is undiluted bleach afterall. Refill with some dissolved baking soda added this time and run up to the faucets to neutralize bleach. Let set a few hours, and dump, and refill with fresh and pump some through to freshen faucets. There are variations. I use a garden fertilizer attachment described in previous threads on the forum, rather than trying to add bleach or soda to a hose; the reason is that I don't think all hose interiors agree with bleach, as the first time I tried it the taste of subsequent water fills wasn't all that palatable for awhile.
Be sure, of course, that you turn the tank fill valve horizontal (and the pump switch off) while filling the tank, or no fluid will go in the tank. Then remember to turn it off (vertical) after filling. On our water bay manifold of valves, the tank fill is top right.
It's been awhile, but as I recall on some coaches you can get at the step cover mechanism by removing an access panel "hidden" next to or in front of the driver's seat. You might explore the carpet between the seat and step cover for screws. You may find air or electric connections under there you can attend to. Ours acted up when it was new, and it was just the deck catching along the guides. Others here are more familiar with your Marquis configuration and likely been there, done that.
-Joel