Problem with 12 Volts lights in trailer dimming

This sounds like a short to ground. Maybe an appliance or other item has a slight short. When it turns on all available power is sucked to ground. Pull one fuse at a time at the power distribution panel. With a multi meter set to inline amp draw test the power required for each circuit. Note: most multi meters top out at 10amp.

Go through connections first. Start at the batteries cleaning terminals, and work your way through others at the inverter. It may also be somewhere in a junction area you don't know is there. Often major connections are made behind light fixtures, or any pannel that will open up in RV's. Twist wire nuts tight etc. Concentrate on grounds. Very likely something is loose, or dirty.

I agree with btd35.. There is a loose connection or a ground that is dirty or has corroded enough that it's nor making a good connection. Start at the batteries, the battery connection to frame, etc. A "short" is a connection between the hot wire and a ground, and will result in a blown fuse or a tripped breaker.

As for the dim lights though, 12 volt lights will ALWAYS dim as voltage drops (batteries are discharged). A fully charged battery is usually around `12.6 volts but when running off of your converter, when plugged into 120 volt power it's around 13.5 volts so they are always brighter when plugged in compared to running from the battery. As others mentioned you may have other electrical problems that are preventing your converter from operating so you are running from the battery so lights will be dimmer.

You fail to mention the power source. Is this problem when using shore power? Onboard power?

The battery and all the battery connections should have no effect on available power when using shore power because the charger/converter is supplying the power for all the 12V stuff and the shore cord is supplying power for the 120V stuff.

Fleeter wrote:
...and sometimes the AC will go off. Even with the AC off the lights on the 12 volt inverter system will go very dim

I suspect "AC" means "air conditioner", not "alternating current". On most trailers, the air conditioner gets it's power only from the shore cord. If this is the case with yours, measuring the battery voltage when lights dim isn't going to indicate much.

I'm thinking maybe there in a problem with the inverter

Here I get confused. An inverter uses battery power (12 VDC) to create 120 VAC - like what the shore cord supplies. If you do have an inverter and it's big enough to run the air conditioner, you will need a _big_ battery bank.

Once we know the power source when the problem occurs, we should be able to do a much better job advising you on where to look for problems.