Battery power lost to trailer.

Like the others I suggest you disconnect the batteries and measure the voltage.

It is also not necessarily true that because they are changing they will automatically be connected to the 12 volt power distribution network.

facory wrote:
Battery voltage without park power is 13.0 - 13.3 volts. Voltage when plugged into park power 14.4 volts. So I know it's being sent from the converter to the battery. However, incoming voltage at the converter measure 2.3 volts.

You are confusing the issue.

The "incoming" voltage on the converter is 120 volts AC.
When unplugged from shore power, it is ZERO and whatever you measure there is meaningless.

If the batteries measure 13 volts with no shore power applied, they probably are good and the power is being lost somewhere down the line.
Start by measuring on the battery cable CONNECTORS (not the actual battery posts). If it's still good, next check the voltage on both sides of the disconnect switch. (Are you sure the switch is in the right position?)
Next would probably be checking that the negative battery cable actually has a good ground connection.
Next look for DC fuses that might be blown.

What is the reading at the battery posts?

You need someone to look over your shoulder and verify the points of measurement.

facory wrote:
However, incoming voltage at the converter measure 2.3 volts. .

Incoming.. you mean Dc.. the battery side? Aren't there separate 12v feed wires to the trailer?

Sounds like it is time for 2 new batteries.. Batteries go bad after awhile, especially if water isn't added and the fins on the inside get exposed to air for too long.

Battery voltage without park power is 13.0 - 13.3 volts. Voltage when plugged into park power 14.4 volts. So I know it's being sent from the converter to the battery. However, incoming voltage at the converter measure 2.3 volts. That's what is confusing to me. It would seem that if it's traveling one way it should travel the other way. Checked all breakers at the power panel and all are OK.

You need to measure voltage at the battery and backwards, towards the distribution panel. That circuit includes a circuit breaker or fuse protecting the battery-->distribution panel wiring.

How do batteries "show" anything. Without a voltmeter your just guessing and so are we.

What's the battery voltage?