We just had a 2nd A/C installed on our '06 at Indiana Interstate RV Service. The new A/C's are completely different wiring than the old ones. Dometic has conversion "boxes" but they do not always work. When Paul Cross at Indiana Interstate could not get any of the Dometic cross over boxes to work, Dometic told him that he would have to replace both of our A/C's. Thanks to Paul's knowledge and tenacity to fix things the right way, he was able to rewire one of the boxes and make the old A/C work along with the new one rather than just replace the old one. Paul also knows these rigs better than anyone else, especially the older ones since he helped start up DRV. I would highly recommend calling him before you start anything like adding a second A/C. You will also probably find that having him install it is worth the cost and assurance that it will work.
Rick and Janice Binns
2006 MS 32TK3, Mor/Ryde Pin Box and I.S., Auto Level-Up
2012 Ford F450 6.7 L Diesel, air bags and 25K Reese Signature hitch (factory installed)