I'm showing up here as a mobile service provider heavily reliant on power out of the service vehicle.
A colleague ran four golf cart batteries in one of his trucks for 14 years. They're easily maintained. Heck, you can get an 'automatic' water setup for them as well.
For my van conversion (sprinter) we built a frame to hang the golf cart batteries under the truck. It's a little bit of a project to lower them and check water. We're currently on our second set. The first set lasted 7 years with ZERO maintenance other than doing our best to stay over 50%. I was terrible about checking water....but they still did quite well.