Is camper parking allowed overnight?

Yes they do. Most Wal-Mart do although their are exceptions to the "rule". There typically is a time limit like 7-10 days. The ones I've seen here are extremely courteous, shop at the Wal-Mart store, and leave the parking lot cleaner than when they arrived.

We always patronize them when we stay in their lots throughout US

I see them there alot. Someine said thry ask permission and usually get a few supplies to show gratitude.

Yes it is & sometimes there are an awful lot of them for being a parking lot!

Yes

Youll have to call that specific location to find out.