how far should the trailer hitch be away from the bumpers:confused: :mad: 
The trailer hitch must be on the bumper perimeter. Other than that, the only rule is that the trailer must hit bumper-to-bumper rather than trailer tongue-to-bumper. How far that needs to be depends on your specific design. See <R18> for full details.
As someone has recently answered you, see <R18-E>
This should be an easy trigonometry problem for someone to work on. Nearly every robot will have its Trailer Hitch mounted to a long, straight section since every corner must have 6"+ bumpers on both sides.
With a 7" wide Trailer Hitch mounted on a straight section with nearby 3.5" bumpers (backing + noodles), someone should be able to calculate the required distance from the Hitch to the nearest bumper to avoid tongue-bumper contact.
Let us know what you get for an answer.
Russ
can they touch the bumpers
I think that the trailer’s tongue will touch the bumper first if they’re butting. That’s a no-no.
Inspectors will almost certainly be attaching a trailer to your robot and swinging it side-to-side to ensure bumper-to-bumper contact (although if the magic answer is, for example, that a 3" gap from the hitch to the nearest bumper is required, we might just use a tape measure instead).
Russ