TTS, I added the mineral oil after 3 years because that is when I had to remove the batteries to clean and repaint the storage area. I had read about the mineral oil trick on the Monaco forum and decided why not as I did not want to do that job again.
A 12 ounce bottle of Thermoil will treat one battery, 6 or 12 volt. I suppose you visited their website. Has lots of info, but of course they are trying to sell the product. I did check my battery compartment this morning and no corrosion and the cells are full. I have a garage for it and keep it plugged in to 50 amp all the time. Of course, I've only had it since June but I'm hopeful the Thermoil does what is it supposed to. So far so good. There were a number of folks on the Monaco forum that used it.
I didn't worry about the warranty on the batteries as I figure if I can't afford a new battery I can't afford the Excel. In response to the charging system Excel uses Charge Wizard which is supposed to be a 4 stage charger. The motorhome had a Magnum ME 2012 inverter/converter which also had a 4 stage charger, if I remember correctly.
2013 Excel Winslow 34RKM
2015 GMC 3500 Denali Duramax 4X4 dually