Thank you!!
Ok, I added my location. I'm in Helena, MT.
I have looked at the Alaska Grow Buckets, and that is one of the systems I would love to try. My hubby is very handy and usually able to whip up these contraptions for me relatively easily. He told me he understands the mechanics of this type of self watering system, and putty one together is on his to-do list

. I keep adding to my pike of buckets, as I get them for free from local food businesses. Yay!
Thanks for the answer on the Pro Mix. I didn't realize that it was compacted. Knowing that, it makes more sense now. I would like to grow inside my greenhouse bus to protect the plants from our almost constant wind. I envisioned a 55-gallon drum to be used as a reservoir, and a whole mess of buckets connected into an array, so that they would all have a constant supply of water in their respective reservoirs.
I suppose the plant spacing requirements for each type of vegetable would determine how many plants I can grow per bucket. So, for example, I could probably fit three lettuce heads in one bucket, versus just one tomato plant, for example (thinking aloud, here, sorry).
I will post a link with pictures of my greenhouse bus...