@100Acre
Battery tender is a smart trickle charger. It will usually monitor the state of charge of the battery, and when it is charged enough to hit x volts, the charger shuts off and continually monitors. If voltage drops below threshold, it starts charging again.
Where as your battery charger will have a 2/10/50 amp setting on it. While they work for charging a dead battery, they typically do not have the proper technology on board to mimic a battery maintainer.
Jump charge on trails.
Excellent question!
15-20 years ago everyone came out with these big jump boxes. Typically $200 and heavy. Had a bigger battery in it than your stock pioneer. Battery technology seems to. Be advancing as fast as one can keep up with it. Now you can buy a 1000 amp jump box that is no bigger than an iPhone.
Noco GB40 seems to be popular. Good reviews, decent price point of $100
If you want help or suggestions on putting together a maintainer or jump box, i can fire up a pc and start sending links. Pita to do thru my ipad.