The code comes up randomly, dealership said fuel contamination. The code started after I switched the fuel filter. Anyone else go through this? And how did you fix it? The code comes up mostly when the trailer is empty.

See if this applies to you..

https://www.gmforum.com/gmc-chevrole...-fixed-301238/

How can I find out if it is the fuel pressure regulator?

You will need a scan tool that can do live data. You will need to monitor fuel rail pressure desired vs actual.

What should my fuel rail pressure be? I believe everything is stock.

Are all these 6.6 DM the same?