I have the same with my Spitfire now and again, they only have a trickle charge on them, I have a cab heater for the winter so that drained the battery, so I carry a set of jumper leads to start it with.
Next winter I am having a caravan charger fitted, my man is a proper auto electrician and is fittng me a charger so the TM battery is charged up as I drive around.
Only cost £30 or so.
Murky