Just completed that myself, George. After much handwringing and consultations among many, I gave up on the lithiums idea. Although I didn't mind having to add distilled water every so often, as the wet cells age and the charger must pour more at them, the gassing increased, and corrosion required more attention than the water levels. The buckles on my hold downs, whether plastic, zinc plated, or stainless, melted within months if not weeks if the coach was plugged in. And tray corrosion became more and more serious.
I finally got the old Interstate Workhorses (U2200, GC2) out, painted the tray with POR-15, and installed 4 Fullriver 6v that a local purveyor offered me a good deal on. I was told that US Battery's versions were American made, and was holding out for those, despite Fullriver having a stellar reputation. But then I discovered I'd been misinformed, and the US Battery ones are Chinese-built as most, including Fullriver, seem to be. The vendor gave me the same price on the Fullriver as the less expensive US Battery brand, and I'd put the task off for years already, so I took the deal.
I called Magnum Energy and they said to reset the inverter to "AGM2". AGM1 is a setting designed for Gel batteries. When I first plugged in, the Magnum Inverter Remote (the LCD screen) read 14.9 volts, and there was concern that was too high. Another call to Scott at Magnum eased my fretting. Only 2 days later an ice storm took out our house power, and for 4 days I used the coach genset to power major home functions, running 12g cords from the ice maker and fridge outlets behind the Dometic fridge. During that time I watched the Remote's numbers range from 13.1 to 15.1 volts. All was okay, and ultimately the readout was 13.1v at "Full Charge". At rest unplugged and the main battery switches off, it stays around 12.7 volts. I've yet to look for the solar controller's dip switches and equalizing mode jumper (switch), as it has rained a lot since the ice storm melt off. I will get to that change before long.
You have an earlier coach than mine, and both my inverter remote and my solar controller are outdated models, it turns out. So as Carl notes, a lot depends on what brand and model of charging devices you have on your Marquis. I can tell you the team at Magnum Energy is really great. The only glitch is that you may have to wait in que quite awhile when calling support; I waited 45 minutes the first time and about 20 the second. But its because they give every customer's issue the time and attention it needs.
Joel