Not that I recommend it, but I know some people run the black wire to ground in the truck bed or frame somewhere rather than all the way back to the battery. Sometimes, I kinda wish I had done that. You can save yourself some cash by buying half as much wire, which allows you to upgrade your positive line (get a fatter gauge). But you have to convince yourself that your ground in the truck bed is good, which I couldn't do.
Well, technically, for me, the negative wire from the camper to my truck doesn't ACTUALLY connect to the battery. It connects to a ground in the engine compartment which I know is a good ground, which eventually gets back to the battery.
FYI, I run 6 gauge between truck and camper with a 50 amp breaker.
Wire isn't really that expensive, buck up and do a proper system. 6ga $26/25ft spool, 8ga tinned $19/25ft spool, 10ga tinned $12/25ft spool. My $.02