I'll have to verify, but thought the ebrake on my rig was automatic when you put the selector in park. I've got a 91 Sun Voyager also on the JD/Oshkosh chassis, but I've only just bought it, and haven't had a lot of behind the wheel time yet.
As for the A/C units, rather than try to match the Mach III, I would look into any sort of rooftop a/c unit. On previous RV's I have had Mach III units, and its all the same to wire a different unit in, if you can't get spares for the Coleman units. I've got a new A/C unit that I'll be putting in place of the one in the front part of the rig, I'll post how much of a PITA it is to replace.
Cheers and good luck with the redo - I'm about to start my own, so its good to hear that you're went well with just those provisios!
Glenn in Tucson, AZ USA.