I would go the 30 LED as it gives a good uniform light without being too bright and as I said earlier, uses very little power (you can have it on for 5 hours and only use 1 AH).

If you only want minimal lighting I try these 24 LED Round Magnetic Work Light With Integral Hook | eBay which I have positioned around the van on anything metallic. They run on 3 AAA batteries and seem to last forever!

And yes the LED can be directly attached to the terminals of your aux battery but for safety add an inline fuse on the positive lead, close to the positive terminal.