Hey there - Campendium has added a page for the free parking area at the casino based on your question / further investigation :)

Its in the front parking lot. Near the main highway. Last time we were there (summer 2019), they were under construction so it was a little tight in the free area. There is a separate gravel area for the truckers, so you don't always here the generators running.

There is free overnight camping in the parking lot of the casino, but there is also a campground if you want hookups and/or plan to stay longer than 3 nights. We stayed in the parking lot for one night and it was very quiet and comfortable.