Covid might have changed this but several casinos used to offer space for RVs. The biggest tragedy is you missed out on Lake Mead! Maybe a 45 minute drive from Vegas there’s a private RV park and also plenty of first come/first serve pads along with fairly extensive boondocking. It costs to get in the gate (unless you have an America the Beautiful park pass, and you should!) Though show up late enough and the gate might not be manned, so free entry.
We’ve stayed at a WalMart once and a few truck stops, but only if we’re desperate. Planning is essential. We’ve found several great community parks and boondocking spots with just a little research.