There's lots of good information on diysolarforum.com (and Will Prowse on YouTube). There's also good info on openenergymonitor.org.
Making your own battery is simple. First, pick your cells and buy them. I chose EVE 280ah Lifepo4 cells. I bought 8 of them, so I have the option of running 2x 12v bank or 1x 24v bank. Then, pick your BMS based off the voltage you want and the discharge rating you need. I chose to build a "DIYBMS", which is very cost effective and feature rich, but there's plenty of options (Daly, OverkillSolar, Batrium, etc).
Then it's just a matter of building a box for it all and assembling it. Since you're building from components, it's easier to make the battery a shape that will work well in your van. While the Gyll batteries are nice, I haven't seen anybody with a rackmount setup in a van. Plus, you can easily keep costs below $0.25/Wh including the BMS.
You'd get more storage and better performance out of a 100ah Lifepo4 battery than your 217ah AGM, and the lithium battery would be smaller than a typical car battery and weigh half as much. Lithium really is the way of the future.