My secret for controlling replacement battery costs is to buy used batteries from the nearest Interstate Battery distributor.
The batteries they sell used are batteries removed from essential equipment where batteries are replaced on a routine, scheduled basis such as hospital equipment.
I've never gotten less than four years use and most often go around six before these need replacing. When the Scamp
is not being used, I have a permanent trickle charger that keeps them up to voltage whenever an extension cord is plugged in.
This is not to say my solution is better than the Lifeline AGM, but I am absolutely sure it is less expensive as I have been getting my batteries for $25 to $30 for a series 27 deep discharge type. Before I buy, the fellow at Interstate has a meter to check the internal resistance of the batteries of size I want to purchase and gives me the best of the group.
I have been doing this for a number of years, since I once owned and operated a business that had a fleet of vehicles. Keeping my operating costs low made my business more profitable. Keeping my Scamping costs low lets me do more Scampering! (I see Scamping is not a real word.)