The Mission Inn is an absolutely AWESOME hotel that's 5 minutes from Adam's.
www.missioninn.com My wife and I actually stay there once a year even though we only live a few minutes from there.
However if you're in your car I'd say Valet. They do have a parking structure in the back. I don't think there is any truck/trailer parking but I could be wrong. The hotel is in a neat downtown area of Riverside.
Are you trailering your car out here or just driving it?