I bought a 1976 F-28 last year and replaced both batteries, then had the same experience as you. I think, but haven't proved yet, that the problem is the fact that the older battery chargers work like this: assuming you have two batteries (you better), older chargers begin charging both batteries but when either of them is full, it stops charging. So if the second battery was more severely discharged, it gets charges only until the stronger battery is full and not after that. So my second battery is always just partly charged and runs out quickly. New charging systems (look for this specific promise on the charger) will top off both batteries, filling one up, turning its charging off and complete the second battery charging until full.
I am trying to work around my old charger's limitations and avoid $$$$ for a new charger, but before I go on any extended trips, I think I'll pop for a new charger/converter/inverter just to sleep better out there in the Bahamas.