Simplest possible solar battery for minivan?

[12VoltInstalls]

haven't purchased anything yet so I could switch to a single large panel. 200W is about the max roof-space available

Look up windyNation 100W monocrystaline panels. Monocrystaline is a tad smaller per watt than polycrystalline. The last pair I bought were $15-ish less money than two poly panels.