Chew and spit time.
Refer to the BatteryMINDER sale in the forum list. Buy one. You can sure use one no matter what.
Cheapest RV battery. Costco, Sams, Wal-Mart. Connect a cheap set of jumper cables from new battery to existing battery.
Have a ball on your outing.
When you get home, charge to the eyeballs then stuff the extra battery in a corner, plug in and connect the batteryMINDER, then forget about it until the next outing. Do check water in between times, though.
When your existing battery dies, you have a battery ready to rock n roll. A coddled unworked RV battery will last 7-10 years or even longer. Coddled means batteryMinder or Battery Tender maintained.
Smart money does not work 3 days a year. Smart money either works or stays ready to work. A four year old worked RV battery isn't exactly a spring chicken.