Just to keep this in view and see if someone else can give you detailed information.
Seems to me that most of these items should not be connected to the same circuit.
I would think, if you have no power to some of these items, that if you look at the Airstream installed fuse box, you should find an 'AUX' fuse that might control some of them. Take the fuse out to test as, if it has been exposed to moisture, it may just have poor contact with the fuse holder.
I would suspect that all these items should not run off the same fuse. (especially the dash air). If the dash air is ACME, it should run thru a fuse and relay probably located on the front outer firewall. There will also be a resister box located in that general vicinity that controls the speeds on the blower. Is the blower working?
The horn: Have you got power to the horn? The electric horn itself might just be seized. This can usually be repaired with patience and a big hammer.
Where is the mirror control located? Have you power to the control? If so, then it is probably a poor plug connection at the ws pillar location. What mirrors are on there. Are they Velvac Turbo Mirrors?
Please call our Technical Service Dept. at 1-800-783-8871 for assistance. VELVAC.
Velvac no longer supply this model, and they used to have an assembly sheet on line, but that appears to be gone. If you need one, I should be able to scan my copy and email it. If you need to take it apart, be very careful as parts are n/a.
Try to be specific when asking for help, as what you are asking for is probably not available unless someone has the original Owners Manual and can scan it for you.