To answer your question, yes you can do what you are asking.
You can purchase the two mating connectors, the proper length of cable and fabricate your umbilical. What is the largest gauge wire the connectors will accept? My guess is you may be able to squeeze an 8 AWG in it but I do not know.
Doing in this way you will have the voltage drop across the wire from the battery, relay, wire to the connector, connector itself, the wire you added and another connector.
I have a question. Do you own a set of jumper cables? Too short?
Determine how short and go to the auto part store and get a second set at least that length. Find two short pieces of 1/2" or so copper pipe. connect one set to the TT battery terminals, you don't need the battery. Connect the other set directly to the truck battery. Connect the two jumper cable ends together by clamping them to the copper pipes.
This will provide the least voltage drop and cost you less with less work.
Just my humble suggestion.
