12 to 6 v battery switchout

In that case you might see your battery voltage look ok but if you put any kind of load on the battery it dies quickly.

One thing that sometimes works in that case is "desulfation". In desulfation you take the battery up to a high voltage, like around 16V, hold it there for a little  then cut off the charging for awhile, then repeat. That works to mechanically cycle the plates and might possibly break the sulfate loose,freeing up more plate area.

So you could try that if you have an old school battery charger that can be set to charge at a high voltage. I wouldn't hold out a lot of hope though after 3 years.

Echogale, maybe try this video, see if that clears some of this up. Look particularly at the ohms and continuity sections. Checking a switch function is really a continuity check, either the switch is acting like a continuous piece of wire or its acting  like  two pieces of wire that are disconnected.

https://youtu.be/ciyWwcKmPC4

One thing that often helps electrical newbies is to think of your electric circuits as if they are water pipes. Voltage is like water pressure,  pushing electricity through the wire. Current (amps) is l

the rate the electricity flows through the pipes. Resistance (ohms) is the restriction of the wire (or load device)  resisting the flow of electricity - skinny wire = more resistance so less amps, just like a skinny pipe will flow less water.

 I suggest caution to start with when 

leaving your batteries on the on board converter continuously on charge. Check the water level in  them frequently (like once a week frequently) until you get a good feel for how much they are consuming. Don't just connect and leave them for a month or two.

When a flooded type lead acid  battery is floating on charge it is continuously converting water into oxygen and hydrogen (called electrolysis). You will see small bubbles in the cells when you look. That's good because the bubbles act to keep the acid mixed with the water in the cells. Otherwise the sulfuric acid,  being much heavier, settles to the bottom, eventually causing degradation to the lead plates in the battery.  But too much bubbling is bad, just consumes water needlessly and risks exposing the plates.

The amount of gassing (bubbling) is proportional to the amount of current being forced through the cells by the converter, and the higher the voltage setting of the converter the  more current.The gassing rate is also effected by temperature (hotter = higher) and the details of the chemistry used in the battery manufacture).  So it's hard to know in advance what your water consumption/gassing rate will be.

What happened to me was that my WFCO converter got stuck in it's "boost" stage  and set it's voltage way up around 15V. It probably will surprise no one here to know that FR hasn't necessarily selected the highest quality converter for their trailers... 

15V is way too high for continuous charging and consumed my battery water rapidly. In just a few weeks I went from completely full cells to barely covered plates. Caught it in time, no harm done as long as the plates aren't exposed . So the warning is, be cautious and check water level often till you have a feel for how fast you're consuming water. 

Re filling new batteries with acid instead of water, that is a one time thing only. Often lead acid batteries are filled with acid at the factory, fully charged, then the acid is drained and placed in a separate container. This is called dry charging a battery and is done to increase it's initial shelf life and to improve safety during shipment  There is no (well, very little) self discharge if the battery doesn't have electrolyte in it, if it it takes months before the end customer gets it installed there is no harm done.

But once you fill the battery one time with acid youre done. It won't consume acid over time, just water, so if you pour in more acid later you'll just be steadily increasing the acidity beyond what the battery is designed for. And that's bad because the higher acidity will cause more internal corrosion and result in shorter battery life. So, distilled water only please! 

A terminal is a place where a wire attaches. The wire terminates there. Thus, terminal. If a wire does not attach to anything, nothing happens. I would like to help you understand. Thanks for letting me know that I left that part out of my previous reply. We all start from somewhere. There was a time when I did not know what a terminal was either or many other things.