Flickering lights

I am assuming the lights are 12 VDC. First question is whether the lights have a fast, more or less continuous flicker, or whether the flicker is occasional. When the lights dim, how long do they dim for?

On the 12 V side, flicker is almost always caused by voltage fluctuations. Have you plugged the trailer into house power? If the flicker does continue, you know it's not the generator. If the flicker is occasional, I would be looking at something coming on, drawing power and dropping the voltage.

You mentioned there is no flicker when the battery is disconnected. This suggests to me that the problem 'may involve the battery charger in the controller.

This is a case where a digital volt meter is close to useless. The update in the counter can miss fast voltage changes. An analogue style volt meter will better indicate a voltage change by the needle moving.

More observations would help
-is the flicker continuous, or only now and then
-does it happen when plugged into shore power, or only on the generator
-do all the lights flicker, or on some of them
-if the battery is disconnected, is there still flicker

Don't forget the grounds when checking connections.

It's a brand new battery and a brand new generator. This problem was happening in the beginning with an older generator and original battery.

It's the light fixtures inside the trailer that flicker. When generator is off there is no flicker, also when the battery was disconnected while waiting for a part the lights did not flicker when running trailer off the generator.

Since a generator does not directly power the lights, I would look first to the battery. Then start with some basic voltage checks. It is also highly likely that the generator is not putting out 60hZ causing the flicker.

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Does the light flicker while on generator? Please advise if light (LED) on generator, the 12 Volt lights inside the TT or if its the 110/120 Volt lights inside the TT.

Do you have a digital Volt Meter? If yes, this tool can be used to test both 12V and 110/120V "current flow". Good tool for diagnosing many different electrical situations as well.

On my setup (a few years ago), the 110/120 volts "light" flicker was because my "generator" wasn't running smoothly. Its engine would pulse up and down and its output voltage would change from 115 to 132 volts. After some diagnoses, I discovered its governor needed minor adjusting. It works great (at consistent 118 volts) now. Perhaps your generator isn't running properly either?