Jay and Bubbly, I made having my small recording studio a priority when I built my bus. The mandolins, guitars, accordions, and vocals are all miked. The keyboard goes direct. I have had three musicians in the front for sessions. There is even room for a fourth. It is a small space, but it works.
For recording,I use the MOTU 828 digital interface. It gets firewired to my old iMac which uses Audiodesk software. We get excellent sound. I believe the key to that is having the ceiling carpeted, as well as the different shaped cabinets and heavy curtains on the windows.
If you want to leave your bus empty, It might be good to hang blankets over the windows and walls. Also, you could cover up some of the ceiling with blankets as well. A metal ceiling will surely be the biggest cause of "tunnel sound" in the bus. You don't have to cover up the whole ceiling. Just enough to break up the waves from bouncing around. Also, have a few carpets on the floor.
If you are going to use a drum set, the drums will bleed into all the mics unless you build a relatively soundproof wall and isolate them. All the stuff we record is acoustic based music. We run the keyboard direct to the Motu. We all wear headphones while recording so that we can maintain a good balance between the instruments. It saves a lot of mixing and mastering later.
I have a few mics. I highly recommend the Samson C01 studio condenser mics. They are less than $100 each. They are just as good as my AKG which costs much more than that. Check them out. They are a large diaphragm element. You'll be impressed with them.
You do not really have to blanket the whole bus. Just enough to stop a continuous flow of bouncing sound waves. You may have noticed the strategic placement of sound foam in recording studios.
Free standing baffle walls are a great idea. You can take them down and store them after the sessions.
I hope this helps.
Here are a few pics of the recording area. Let me know if you have any questions.
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