I am considering a grow bag idea -- there are various ideas --
a mobile garden
how to make with the gardening cloth
basically, it is polypropylene gardening cloth that is cut and sewn or hot-glued to form a box or bowl... filled with potting soil that contains coconut husks, peat moss, or hay which allows the wicking of water through the bag.
The versions I'm seeing are larger than my idea, which is a small grow bag that can set in a small dish of shallow water (or for a quantity of bags, a tub of water) which allows the air to circulate around the bag, pruning the roots so they don't form root balls. Plants draw water as they need it, I take it. (Bonsai trims both roots and branches to keep them small... which I want to experiment with to see how something will fruit.)
I have seen potatoes grown solely in a bale of hay, so my idea is to try a larger bag that contains a lot of hay in the mix, to reduce the weight. Tomatoes, peppers, beans, etc of the bush variety can be pruned so that they don't become unwieldy to pack back into the van. I might possibly have a couple yards of fencing to wrap around them at camp to support them in wind. I'd only have one of each kind, and smaller salad garden bags.
The general idea is... when it is time to leave a spot, I give the bags a day without any water to reduce the water weight... pack them tightly for the next trip (they will survive a day or two) -- and unpack, allow them to air, and water them when they are out in the sun again. It may look like a garden but it is food... fresh food that does not need refrigeration. They would be in the sun during the day, and on my front seats at night... away from patrolling animals.
I forgot the other part of my idea -- the olla pot -- which is an ancient watering technique that reduces the evaporation of water, but allows the plant to draw water as it needs it. The roots actually adhere to the pot in the ground and suck water from the pot. There are various ways of monitoring the level of water in the pot, by a stick attached to a piece of styrofoam in the pot that will sink when the water level is down.
a guy making the pots
You don't need two clay pots, only a pot and its clay saucer, especially if the grow bag is not very large. Large clay pots would work best for an in-ground garden, where you would plant around the pot. The clay pots you need are those fired at low heat, and they will be cheaper. If it has been fired at high heat, it will not seep water.
Both the olla pots and the "wicking" technique are ways of preserving the water by preventing evaporation. The plants only draw the water as they need it, and it is not left to evaporate. Watering plants from the top does not provide the water properly, and you can over-water or under-water.