What about a small trickle charger readily available from Jaycar and cheap.
Or you can spoil the your Dad for Fathers Day, and buy a decent charger that hooks up to the trailer directly on dedicated lead wires.
They are available to charge and then switch to trickle automatically !.
Good ones mount to a wall, you hook up and forget, it takes care of the rest.
To easy.
Some condition a battery to maintain performance and this sounds like the type of charger you may need if it's not used so often .
Cheers