You really need to get NEW cables. Whatever you do DO NOT put a Butt Splice in.
I just re-did my Battery/Pigtail wiring and posted pics onlne in the Travel Trailer Forum HERE
Batt WiringBe sure you put in the appropriate size inline fuse very close to the battery and I really suggest a fuse holder and weatherproof box as I installed.
Building your own Batt Cables is easy but you need the proper style crimper if you go above #6AWG And I agree with Using nothing but MARINE GRADE cable.
Also apply heat shrink on all your new batter ring lugs. This will help in reducing the creeping corrosion that makes its way up Batt Cables whose wires are exposed to the elements.