Add a second battery to your Vanagon! We have been using these in-house as a lower cost alternative to increasingly expensive domestic brands, and have gotten excellent performance from them. They are totally sealed and maintenance-free, and fit in the compartment under the driver's seat!
IMPORTANT NOTE: As with any deep cycle battery, it is important to ensure that your new battery is fully charged prior to using it for the first time. Batteries discharge naturally over time, so make sure yours is topped up completely prior to installation!
We recommend reading our article, "Batteries: How to Keep 'em Happy."