Alliance Coach  are  going to reconfigure the 24,000btu RVP # 46515-811 $3257 unit to fit which Good Sams approved as a replacement .They have used almost all of the 8 hours Good Sams allowed for this job.
We are assured they will work with with us  on the hours billed as they want us to be happy with the final outcome.
It was decided rebuilding the old SCS unit was not a viable  option.
I have since been told that Beaver Coach Sales  rebuild   old SCS units for $3000 ,using your unit as a core.  However it is obvious the New RVP unit is far superior in performance even though it will cost to install.

Photos

Thank you Gerald for shrinking these photos and posting them for me.
Work started on Thursday morning ,hoping for completion by Thursday June 2nd. Memorial Day weekend means no work Friday or Monday.
Looking at the photos   the return air duct (black rectangular shape with 2 shiny screws(filter access door) and the cold air duct into the coach (top right small square duct) are at opposite ends of the new unit.  

Wednesday 1st June.
  After starting work last Thursday 26th May ,Alliance Coach have modified the return air and cold air ducting . Modification of the forward bulkhead is required to allow the RVP unit to swivel into position. A new   RVP thermostat is on order as the SCS unit is not compatible .  

Modified return air blower duct

Thursday June 2nd.
  Alliance Coach have removed the forward wall/bulkhead that supports the tray  to allow the RVP a/c unit to swivel into position. They now have to fabricate a thinner support bulkhead as the RVP unit is longer than the old SCS unit. Once the bulkhead is installed they will hook up the modified RVP  unit to the old ducting , connect the 2 110v ac power supplies and install the new RVP thermostat.  

Friday 3rd June
Alliance Coach  are fabricating new  ducting.In the bottom photo the black square metal box, above the RVP  with 2 ss screws on its face ,is the return air filter tray. The screws undo and the plate folds down to allow the filter to slide out.

Monday June 6th 2011.
  Alliance Coach has completed the ducting.  Looks good.  Presently wiring the RVP in.  Looks like the new thermostat lying on the coil of grey wiring in foreground.  The new forward bulkhead is on left.

Wednesday June 8th. 2011. Day 9 .
  Alliance Coach have told me the following:  " The a/c is ducted, wired and running; it is really cold; in a 1/2 hour while it was 95 degrees outside the coach was cooled down to 65 degrees. They also said the RVP unit is very quiet.
This might interest some of us who have to let the SCS unit run all day and night, to keep the coach at 80 deg F plus when its over 95 deg F outside.
   They could not reposition the thermostat for a faster installation as the Hurricane Furnace wiring runs inside the slide wall to the same location, which meant they had to remove the wall to run the new thermostat wiring.  The Technician has to reinstall the wall, after installing the thermostat and button things up .

Thursday June 9th .
  The final day reworking and installing the air filter access cover ,the condenser coil cover and exhaust  flex  ducts.

Friday June 10th. 2011.

   Job completed early today. We will pick up coach on Monday 13th June.

It would be good to know the final outcome of this project, especially in view of all the negative comments about being "ripped off" by Alliance Coach. I would venture a guess that the final cost was much greater than the original quote of $4000. They must have worked on the job for at least three weeks. How about an update.

Thursday 23rd June 2011.
Louis, you are correct.  After 57 hours work, not including the 8 hours Good Sam allowed for R&R of the old SCS unit, the new RVP 24,000 BTU basement air conditioner is installed and running.
The main problems were, having to alter the units cold air output and the return air inlet from the coach. After the RVP was installed,  Alliance had to  R&R  the unit to add bracing to the cold air blower squirrel cage, to stop the blower wheel from touching the side of the housing.  This was done at no cost, under the 30 day warranty, after consultations with Coleman.
It meant a return visit to Alliance Coach  for 3 days.  
We are now in Utica NY, and the unit is running amazingly well.
Alliance Coach was very good to us as far as the final bill was concerned.  We settled for just under $3000, which included the $500 deductible for Good Sam, who picked up the bill for the new unit and the initial 8  hours labor,for diagnosis and removal of the old SCS unit.  
Alliance Coach is well aware many of us are watching this site and might find ourselves needing this upgrade in the future.
Now they have this experience under their belt, I would recommend anyone needing this upgrade, go to Alliance Coach.  Just be aware some coaches are different from our Beaver Marquis Amethyst, and might require slightly different configuration.
Overall, a very frustrating experience, but the outcome is well worth it.
The new Coleman RVP unit is way more efficient than the old SCS unit.  It cools the coach down after a hot days run in less than 10 minutes.  At 95 deg outside temps, it keeps us cool at   77 degF.or lower.
It is much quieter than the SCS unit, inside and outside. Parked next to another RV, we no longer blow them off their site or drown out their conversations.
If anyone want further info, please email me at: svpelican1@hot mail.com