I Have used Walmart many times, the advantages are...
they are almost everywhere,
great place for restocking while traveling,
very rarely get hassled, ( on the ones that do not ban camping)
Well lighted (not so good sometimes)

Some disadvantages:

Noisy all the time from the traffic, trucks loading unloading, semi trucks parking for the night running their rigs all night, parking lot sweepers

Sometimes, parking lots are very crowded late in the day

Open 24 Hours

Trash everywhere

I have found that other places are just as good or better

Sams Clubs (own by wall-mart) They close at 8 pm

Costco, very rarely used, usually very clean

Cabellas , rv friendly, many have dump stations, designated rv spots for overnight, they allow 2 night stays free.

Bass Pro Shops ,

Cracker Barrel , (disadvantage is small parking lots for lager rigs)

There are many more, when in doubt, ask

Very good web site .. Cheers