Lights get bright and then dim

Probably loose ground connection or dimmer switch is going bad. Electricity is like water. Batteries are the source, wires are the conduit, switches allow passage, fuses keep the smoke from being let out of wires. It's easy, start at the battery, you have power from there to the fuse panel, from there it goes to the light switch to the dimmer to the lights to ground. OOPs I was thinking going down the road.

Sounds like your 'converter' is failing.
When connected to shore power the converter takes AC power IN and puts DC power out.
That DC power is used to supply all dc demands and recharge the battery

Converter DC Voltage output should be steady (depending on demand).
When converter starts failing the DC Voltage will fluctuate.

AND this dimming/brightening of lights can be due to a bad battery......shorted/bad cell

So you need to check each battery cell 'specific gravity', check battery voltage/load test and converter DC output with battery disconnected....voltage S/B 13.2V

Time to buy a digital volt meter and learn to use one......every RV owner should have one and know how to use it.