I wouldn't drive back just to get gas, but we often plan a stop on our way to other activities on the island, away from West Maui (e.g. Haleakala, Hana, Iao Valley, etc.). It can be worth it to go to Costco to buy more food if we're doing some morning activities in South Maui, like La Perouse, Wailea, etc.). We don't usually stay just in Ka'anapali and Lahaina the whole time.
If you don't venture far from Ka'anapali, you probably won't need gas until you leave anyhow.