I wouldn't even think about splicing that 7-way trailer cable except to get home on. Go buy a new one. ETRAILER.OCM has two length sizes available on their web site.
I have seen them for sale two at some RV places...
Just be sure to write down the 7 colors and make sure the one you buy has those colors - a couple of different suppliers might have different colors used.
If you have same colors then its easy to install a new 7-way cable. Then you just use the large twist on connectors in the electrical junction box under the trailer "COLOR for COLOR"
I think this is the third battery terminal melt posted this week. Must be a full moon up there...
just my thoughts
Roy Ken
