Can I use a busbar as a ground in my travel trailer?
You can use a bus bar, yes.  

But you should never assume that the entire trailer is bonded together in such a way that you can use any of it separately as a connection to the bus bar.  

Think of how many pieces it takes to build the trailer.  Screws rust and aluminum reacts to the oxygen in the atmosphere to produce resistance.  That's the last thing you want in a ground system.

So if you use a bus bar, be sure that your "things" are grounded directly to it, and that you have a good thick copper wire connection between the bus bar and the service entrance.