[ZekenSpider]

My thoughts are:

In any TrailManor that I have worked on, the 12 volt side of the TM, tow vehicle(if plugged into the TM) and converter output are all tied directly to each other. All of the positive wires are connected to each other (some through plugs and through fuses) and all of the negative wires and frame grounds are tied to each other. Look upon the DC system as consisting of a positive Buss and a negative Buss (Buss is just a fancy word for a common connection. There is a positive buss and a negative buss). Weird things can happen if any single wire becomes disconnected it's appropriate buss.

The most common malfunction occurs when a ground (negative) wire gets loose or corroded and is no longer connected to the negative buss. The TM frame is the primary negative buss.

A converter malfunction should have no effect on what I call this TM 12 volt Buss unless the converter malfunction (if any) is drawing current from the 12 volt Buss. It is clear from your tasting that the converter charges the battery so that means both the positive and negative wires from the converter to the battery are connected.

Your test also showed only 1.5 volts DC at the disconnected battery leads with the tow vehicle connected and running. This tells me that, at least, one of the battery leads is not connected to the part of it's DC buss (probably the negative and the frame). I suspect the TV negative is not grounded.

I would look carefully at all TM frame connections. The converter negative output should be tied to the frame and the tow vehicle cable negative should also be tied to the TV and TM frames. Do not use the hitch ball as a negative (ground) connection.

These are tough problems to track down but a serious clue is that "it used to work but now it doesn't". Frame connections very often become loose or corroded. First use your voltmeter to check for voltage from all positive test points to the frame. I think you will find the tow vehicle negative cable is not connected to the frame somewhere. I found mine in a loose wire nut inside the refrigerator outside lower access door.

Hope this helps,

Jerry