Hi Blip.
I`d stick with the standard 100Ah battery.
If your just running lights, I`d be purchasing LEDs for a fraction of the cost of a bigger heavier battery.
LEDs throw out just as much light, but only use around one tenth of the power.
I`ve converted my van over to LEDs and use around 2amps per hour with all my lights on.
I was originally using around 20amps an hour with all lights on.
At 2 amps per hour, my 100 amp hour battery has no problem running these lights.
Go to ebay and type in "Jayco LED lights"
At a minimum, I`d be getting 6 x sets of 36SMD LEDs. These are a good choice for the main interior lights inside the van
A 48SMD LED is a good choice for the exterior light
A few sets of 12 SMD LEDs are a good choice for the bed lights
All these connect to the Jayco light fittings. They come with 3 different attachments. The little wedge one is the one that simply clicks into the jayco light fittings. It`s all a matter of pulling the old ones out and pushing the new ones in. They connect using double sided tape, but mine needed a blob of silicon to provide a bit more stick as the double sided tape on a couple did come off.
Heres a link below. These are what I purchased and installed.
Your 100amp hour battery will have no problem running these lights.
http://www.ebay.com.au/itm/Jayco-LE...ccessories&hash=item3cc537232f#ht_1023wt_1141
At a minimum I`d purchase a few sets of 36SMD LED lights.