battery water going low on only 1 battery

Glad you are checking your batteries...

Two things to look at...

What is your converter/charger unit. Being an older trailer chances are it is a single mode 13.6VDC charger... You really should replace it if this is what you are using for a more modern multiple mode smart mode charger type. Progessive Dydnamics or IOTA brands are tops... I would not get a WFCO product as they are known for not going into smart modes when needed. It is well known that having 13.6VDC being applied to the batteries for long periods of time will cause boiling out fluids. This will eventually cause the battery to have an internal cell short which is not recoverable and require replacemnt.

Another thing would be to make sure you are using a balanced connection to your two 12VDC batteries wired in parallel.

Use a wiring scheme like this using equal lengths of cables inside the battery terminals... This insures both batteries demand the same DC current when being charged...



Continue to watch your battery fluid levels on a tight schedule until you make corrections like discussed here...

Just some of my thoughts here...
Roy Ken

"Nehhhh"

Check charging voltage. If above say 13.4 and it's a 75 degree day the voltage is too high. Battery #2 could have an open cell.

If charger voltage is normal then suspect the thirsty battery as having gone bad. Test it.

If the "good" battery is more than a year-and-a-half old you should replace both. You can have a hundred batteries on a charging line. Twenty can be bad. The good batteries will not suddenly start using a lot of water

If you play
Eat one now save the other for later...

The old baytery will eat the new battery. Fiscally a poor choice. Boiled-down there isn't really advantage in testing unless the batteies are recen

If you have the two wired so that one has the "load and re-charge" wires on it while the other is "downstream" then that might explain it if the one with the load wires on it is the one losing water more.

That would be from it doing more of the work than the other.

Or it could be just going bad.

a bad connection. or bad cell in one battery.

remove both batteries.. charge them.. then take them to an auto parts store..

request a 100 amp load test be done on both batteries.

if both batteries are OK, check all connectors and cables.