There have been posts on a few forums about replacing the Fedders unit in a Roadtrek with an 8000 BTU or sometimes a 10,000 BTU LG branded unit from Home Depot. Some have mentioned getting units that fit from Walmart. Home Depot is a good place to go because they should have some units on display that you can measure.
This link: http://www.classbforum.com/phpBB2/do...file.php?id=80
is for a PDF file that has instructions on how to replace the air conditioner in a Roadtrek. I think it is for a newer model than yours but it will give you a good idea of what needs to be done.