We hook up both our deep cycles to two separate Battery Tender's. Last winter they were on the charger from late October to late March. I checked once a month and never had to add any water. This year I will check and fill when I pull them from the RV, hook up to the Battery Tender and leave them alone. Hoping for the same as last year. Your results may vary.
I started off using a Battery Tender on my 1991 Harley Fatboy. Two years in a row it "boiled" or evaporated all the water out of the battery.
Visited with a Battery Tender Rep in Sturgis and he recommended only hooking up the battery tender for one week a month. So that's what I've been doing for more than 10 years with no further issues.
This past year I changed out the battery in my hydraulic dump trailer to a new, big, Optima. I purchased a new Pro-Logix PL2320 chargers for the Optima. It has lots of options and seems to do a better job of disconnecting itself from the battery when it reaches 100% charge.
Thinking of trying the Pro-Logix on the Voltage batteries this Winter.