It was more expensive than I expected. But, it really makes it super easy to top off the batteries and you can’t overfill them. Just pump the bulb until you can’t pump it anymore.
Here are the links to them on Amazon.
Flow-Rite RV2000 Pro-Fill RV Edition 2 Battery Kit
https://www.amazon.com/dp/B001FCAXT0..._kNSQCbK29736Z
Flow-Rite RV2020 Pro-Fill/Qwik Fill Hand Pump
https://www.amazon.com/dp/B001FC79EW..._DRSQCbGTVKJ2F