Can you park an rv at this walmart for a few hours

Yes

Is there a 24/7 store around SLCounty?

Yes, but you have to be careful about it. You can park and sleep but you can't just camp out in their lot.

You can! Apparently you have to be careful about overnight parking because its technically not allowed. You'd be safer parking overnight at a 24/7 Walmart, if that's what you're looking for.