The is nothing wrong with using chassis/frame for earthing (grounding)... However for simplicity it may be easier to run both negative (earth) and positive (power) lead directly from fuse panel to LED lamp or USB "port" or any other 12 volt sd device.... Its really personal preference.... Although in some cases it may be more difficult to have decent "earth" (ground ) on chassis if its a bolt together frame that is painted before assembly and other reasons as loose bolts and old age and corrosion...
My chassis on all my trailers are always grounded (earthed) to on board battery and also when connected to tow vehicle ( I also use charge while towing) ....
The key to all this is making good "fault free" connections and using a corrosion preventive grease at ever crimp or bolt/screw connection....
I support using a fuse pane that also provides a "connection" or grounding or return path (not necessarily this brand below but this type concept)

Note it has 12 fuse positions (ACT Type) and 12 "ground" positions..
Dale
Lives his life vicariously through his own self.
Any statement made by me are strictly my own opinion.
You are free to ignore anything I say if you do not agree.
