I'll second the motion, to start looking through, searching youtube videos related to this. It is not realistic for anyone to provide you a tutorial on all this via an online forum, and its very unlikely anyone on here is going to know specifically how your rig is plumbed. Especially considering how old it is.
I would just do some searches on youtube, watch some videos, learn all you can that way. There are soooo many videos out there that explain how to do most anything with RVs. You are definitely on the right path, by getting a mobile RV tech out there. Hopefully you get a good one, and they will be able to help you with these things, and help you learn how these things work.
My rig also has the cables connecting the handles to the dump valves. Not crazy about that approach, but is necessary in many cases. No doubt sometime in the future, I'll have issues like what you're having with the cables, handles.
The more you can learn how these things work and get to where you can handle some repairs yourself, the better off you will be.
