[bestgenman]

Jon, an 89 model  that I once owned, had four split systems and you are correct about the cap tube expansion design.  One  of the evaporators plugged at the cap tubes.  I cut them out and replaced with an expansion valve.  My local refrigeration supply helped size the valve as I had no idea how to proceed.  It was a lot of work but performed perfectly.  I may still have the info if you are interested. 


I did learn that silver solder is not needed and was glad about that since the inside of the copper tube will flake unless an inert gas is inside the tube.  That flaking will plug the new cap tubes.  I used a solder that works well with a propane torch and no flux, your local refrigeration supply house can tell you which silver content to use.


The condensing units do not do a good job of moving air since they are "couped up" in the side boxes.  your idea of using a roof top for parts is perfect!  The problem with the evaporator is the size may not fit your space.  


I built two condensing units from roof tops, here is the design:  Using the squirrel cage blower that was for the evap,  it now becomes the fan for the condenser .  Swap the condenser  coil to the evaporator location so the fan blows  the heat down through the bottom of the coach box, leave the compressor in position and re-solder to the cond coil.  Cut off the motor shaft that drove the propeller fan since you won't need it.  the efficiency increase is dramatic.  The electrics stay in place as does the fan, compressor, & base of the roof top (you do have to cut it down to size to fit into your coach box).


I do have several parts left over from that project so let me know if you're in need of something and I'll check the stash.