I assume you are running dedicated positive and negative cables from the battery to the winch. If you use the HF recommended 6 gauge for 25' in each direction, you will be seeing 1.8V of voltage drop. Don't forget that the 6 gauge wire HF uses is for a 3' run from a battery If you run just one cable from the battery and return through the frame, you are on your own. If nothing else, your voltage drop will be even higher.
Is it really necessary to run the Neg. (Ground) cable all the way from winch to battery? Can't you simply ground the winch to the frame. Provided that the battery has a large enough conductor grounding it to the frame to handle what the winch would pull. The reason for the question is that my...
A small winch like that draws a current similar to a welder. When I weld on a vehicle, my standard practice is to put the ground as close to the weld area as possible to minimize the possibility of damage to vehicle electronics.
Just a suggestion, but price out 50' of 6 gauge wire (undersized IMO), a quick connect coupler, and four lugs (plus a big cable crimper if you don't own one) and compare it to a battery. I run a 9000# winch off a Group 75 battery (simply because it came off a car that I was hauling to a scrap yard for a friend) and I've never run out of battery, They sell for less than $100 new, and about $30 from your local pic-n-pull. Bolt the battery down, spatter some latex paint on it to make it look really old, and it will probably be passed over by midnight-auto shoppers