Not in a Crusader...... but I installed a 2nd ac in our Sabre's bedroom this weekend, it was factory prepped for 2nd ac with a separate thermostat. I was going to get a dealer to do it but got frustrated with them quoting me non-ducted units when I told them every time that I wanted ducted and already had the ductwork there so I decided to do it myself.
Its an easy project, you'll need some help to get it onto the roof and set in place. I backed my truck up to the front cap and had two guys in the bed push it up while I was on the roof with a rope, we taped cardboard to the bottom so it didn't scratch it up, went real smooth, hardest part was removing the existing vent.
I'm assuming you are taking out an existing roof vent and putting it there, do a little bit of investigating and find the wiring first and make sure you have ductwork taps already, if it is indeed prepped for a 2nd ac everything should be there. I had to remove a ceiling mounted speaker and fish around for the thermostat wiring, once you do that get the model number of your current ac and call the manufacturer and they will tell you what you need then you can download an installation manual online, and plan it out.
btw, mine was a Dometic, they were very helpful
2013 Sabre 32bhok-6
2012 Ford F250 6.2