I installed a 30 amp breaker in a square D box to run my RV. It looks like the neutral bar and ground bar are the same. Other breakers that are in place work fine. When I turn the 30 amp breaker on the lights go bright and then the 30 amp trips. Any suggestionis

What kind of wire did you run?
What kind of breaker is it? Single pole or double?

I have the suspicion that you installed a 240v circuit to a 120V travel trailer receptacle.

Can you post a pic of the panel?

It is a single pole breaker and it has 120/240 on the front of it. The wire I am using is 12/2

One problem is the cable. A 240Vac, 30 amp circuit requires # 10/3 AWG conductors. #12 AWG conductors are only listed for 20 amp.

Next, how far from the panelboard is this RV?

In order to feed 240 Vac from the panelboard, you must use a dual 30 amp breaker. A single breaker can only supply 120 Vac.