From jct I-90 (exit 98) & Baseline Rd: Go 3 blocks N on C St, then 1/4 mi on US 14/16, then 500 feet S on A St.
Self-serve Potable water and dump station is $5 cash. Overnight stay is $20 with electric (don't know about Easter on site) and that includes dump. Appears to be first come first serve for overnight stay self-contained RVs only. Right next to the gas station which has a better price than Wall, SD.