LEDGER65 - Just insure your 21FPS trailer is using a smart mode converter/charger that uses the three or four mode charging events when charging your battery when plugged into shore power.
These smart mode charging techniques will look after your battery automatically for you and help eliminate the boiling out of the fluids from overcharging etc...
Just helps you make your batteries last longer. You can do the same thing with a smart mode manual battery charger but why not just let the on-board converter/charger do this for you. Find the model number of your on-board converter/charger and enter this into google search and it will tell you what you have or list on here. The newer trailers maybe year 2012 and up usually have these already installed by the manufacture.
Sometimes it may just be a battery going bad on you - I always try to watch things more closely to catch things before they happen. might be able to save a few bucks from not having to replace a battery. Good planning makes for successful camping trips hehe...
I have several batteries on my setup and I use the PD9260C 60AMP model converter/charger unit to keep my batteries re-charged.
Roy Ken