Short answer ... Yes, you can safely run both 12v DC and 120v AC grounds to the frame of your trailer if the remainder of your wiring is designed and done correctly.
This subject comes up from time to time and often incites a myriad of discussion and information
both good and bad.
Here is a link to a comment that I made earlier on a similar subject. Sorry about using the link, but I have grown too lazy to retype my reply. I'm sure there are many others with similar questions and there is so much on this subject that a search can be frustrating.
