The Sam's that is nearest to me, only open a couple of years, has a parking lot arrangement that is definitely not RV friendly. The parking lanes seem narrower than normal and there are high concrete curbs at the ends, even the middle aisles. If you pull in there in even a modest size RV you will run over some curbs before you get out. Forget getting to the gas pumps as there is the same narrow exit lane with high curbs.
The only way to overnight at this one is if they would let you park where the trucks unload in the back, sounds restful.