Or if you want to go old school for $13, buy this.
Amazon.com: Plews / LubriMatic 75-030 2 Quart Professional Battery Filler: Automotive
Just another option for you. I use this about once a month, just push down on the lip of the cell until water stops, it is full, move to the next cell , repeat.
06 Hurricane 34FT WH W20 Chassis 8.1L 120K, Steersafe, Koni Shocks, DIY Trac Bar, Tri-Metric 2025RV Battery Monitor, 4-6V Batteries, Scan Gauge 2, Crossfires, 735W Solar Morningstar MPPT-60, KVH R5 In-Motion Sat, XM Radio, 07 Chevy Malibu Maxx Toad, Falcon 2, Brake Buddy, Escapee