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Have you checked the voltage at the battery?

"New converter installed."
Was some wiring 'disturbed' when the converter was changed?
I can't thing of much, if anything, about the battery itself that would result in these observations, you checked the voltage with the voltmeter right, at least 13.6 volts?

This is my sons TT that I ma helping with. I assume voltage is good at battery...but we all know what happens when we assume ??. Next step check voltage at battery.

Yes, check battery voltage with unit unplugged.
If the problem started after the converter installation, it might be hooked up backwards. Lights don't care about polarity. Check reverse polarity fuses.
New converter could be bad.

in my Flagstaff the 12 volt side of all of those is on a single fuse, labeled appliance

I would double check the fuses and be sure the connections to the fuse panel are tight. Does your unit have lights that come on when a fuse is removed or bad? if so you probably have one that doesn't light and that is probably the appliance fuse, start tracing from there

dmcgiffin wrote:
This is my sons TT that I ma helping with. I assume voltage is good at battery...but we all know what happens when we assume ??. Next step check voltage at battery.

There is nothing to indicate it is the battery .....
Check the voltage is a level charging voltage where there is a problem, and work back from there (if you are lucky the fuses may actually be accurately labelled). It's most likely related to the new converter install. If it was backwards you should have blown the reverse polarity fuses. If the converter is not working then why would the lights not work yet other things do work .....since you seem to have incandescent lights they would work even at very low voltages (with a different colour).

Once you verify the converter was installed correctly... I would be checking the 12 volt ground bus.

Does the trailer have a battery disconnect switch?

It looks to me that the battery is not in line. Reason the indicator levels on panel do not function along with 12v appliances. A incorrectly wired converter might leak some current to light some 12v lights.

How old was the old converter?

Some quite old converters were set up to switch some of the circuits in the RV from battery power to converter power and had a separate battery charge output. Modern converters just have everything hooked up together in parallel, so that the converter output both charges the battery and powers the 12V systems and there is no 12V transfer switching involved.

If the old converter was of the switching type, it's likely that the 12V fuse panel has two (electrically) separate sections, one for circuits that are switched back and forth and one for circuits that are always powered by the battery side (sometimes called "unfiltered" and "filtered" or some such, the battery acting as a filter for the output of the converter). If the two sections of the fuse panel are not jumpered together properly when upgrading the converter in such systems, assuming the fuse panel isn't replaced entirely, it's quite possible to end up with some circuits no longer connected to 12V power.