I put 4 - Trojan T-105's 6-volt true deep-cycles in my Seneca last summer when my O.E. Harris batteries went bad. I believe before I bought it the unit it probably sat dead over a winter and the batteries froze. I started having issues so I switched them out to prevent a failure while camping. I could have tried making a warranty claim, but I knew I wanted something better anyway. I might have gone AGM batteries, but I had a "connection" and got the T-105's for $75@, plus I needed them right away and the AGM's would have to had been ordered.
They fit very nicely, slightly taller but not an issue at all in my unit. Obviously I had to re-do the connections; I have two batteries in series (to make 12-volts) and then each pair in parallel. If I have a battery go bad I could quickly take that pair out of the loop and stay in business.
I also went with the Trojan single-point watering system. Makes it very easy to water the batteries without trying to move the cables and get the caps off, then spill water (and overfill) almost every time. I just attach the quick-disconnect bulb pump to the fill line, stick the suction hose in a gallon of distilled water, pump till it stops, and each cell is filled to the perfect level. No muss, no fuss.
I am very happy how it went, and I know with proper care these batteries will out-perform and outlast the Harris units it came with.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST