The OP wants to change the trailer to the NEW style industry-standard Pollak 7-pin flat, which I'd highly recommend in case he ever needs anyone else to be able to tow the trailer.
I'd be careful and chase each wire since the 1996 and 2000 Jayco Designer 5th wheels I owned came with the Pollak 7-pin flat. If the Jayco you're looking at has a 7-pin round, that's non-standard for the Jayco and there's no telling how the previous owner had his truck wired.
Here's Pollak's standard for a 7-pin round:
Here's Pollak's standard for a 7-pin flat:
Rusty
