Lights dimming

The simplest things can be so difficult sometimes.

You are looking for 13.2 volts on the battery. Then disconnect the battery and check for voltage in the RV. Then check the main converter fuses. Bench test the converter. Install new converter.

If you charge from a generator make sure the replacement is a Progressive Dynamics or IOTA.

turn off shore power
start checking battery cables,
look at the negative cable where is bolts to the frame

then open the cover on the converter and check all the wiring connections

also this is TT ? right ?

so follow the positive battery cable from the batteries, find the DC thermal circuit breaker
looks like a silver box with (2) wires attached
common size is 40amps, replace it
temporary test is unbolt the wires and bolt them together, if power is OK, the thermal breaker is bad

Who hooked up the two new batteries?

The lights work off the battery(s) so I would check the voltage there. It should be around 12.6 volts when not charging. Were the new batteries charged up with proper water level?

Sounds like CW might have sold you something you really didn't need. Easier to take your good money than to find the problem. Apparently the batteries weren't it.