Thanks for all the replies guys, I contacted 2 auto electricians this morning and they both said that they all go back to the battery but the only reason they do it that way is to ensure a good earth, so looks like if I can get a good earth then there is no other reason why I can't just earth the neg to the body which most likely will be via the tow bar.
That connection should be easy enough to test so looks like I will do it that way as a single cable should be much easier to route through, now just have to work out how to get the cable through to the back and under to the tow bar.
I have a length of single 10mm2 cable, a 50A circuit breaker and a couple of Anderson plugs laying around in the shed so can now put them to good use
. My reason for it is mainly for the air compressor and to a much lesser extent, to powering a electric/hydraulic brake system if I ever get around to towing a big trailer.
Cheers