Option 1: Lewis and Clark Interpretive Center
Open 9 to 5 dailly; looks like has good RV parking ; $10 per adult fee - gets you into Fort Mandan too; https://www.history.nd.gov/historicsites/lcic/
This is the primary Lewis and Clark Interpretive Center in North Dakota, located approximately 38 miles north of Bismarck and about 40 miles from Douglas. It features a museum, exhibits on the Corps of Discovery, and is adjacent to the reconstructed Fort Mandan.