Yeah, it's been a problem that's been discussed before on this and other RV forums. The sheathing on the cable is evidently routed too close to the exhaust manifold on some of the F3 chassis.
Read through these three threads and it will give you an idea of what causes the failure and some solutions:
04 Damon Shift cable
(some interesting photos with post #5)
Transmission cable failure
Help gear shifter cable f53
This sounds like the most promising solution so far from the last post in the first thread by tiffy2000
(maybe PM him to find out more?) as it pertains to fabricating a cable ...GO HERE
Hopefully, others will chime in and maybe have some more up-to-date information as to solutions.