Here is a list of the Walmart's in Dallas taken from this Yahoo Group:
https://groups.yahoo.com/neo/groups/walmartrving/info and the most recent status of overnight parking.
The Walmart's with "NOP" in the address are flagged as No Overnight Parking. As always call the store and ask permission to park for ONE night only. It is not a campground. If you are looking for a multiple night stay, the use an RV Park.
Super Wally 6185 Retail Rd Dallas TX 75206 NOP (972) 656-2196
Super Wally 1521 N. Cockrell Hill Rd Dallas TX 75211 (214) 330-7249
Super Wally 9410 Webb Chapel Rd Dallas TX 75220 (972) 629-0007
Super Wally 7401 Samuell Blvd Dallas TX 75228 (214) 319-2616
Super Wally 200 Short St Dallas TX 75232 (972) 232-6400
Super Wally 3155 W. Wheatland Rd Dallas TX 75237 (972) 709-1400Super Wally 7075 W. Wheatland Rd Dallas TX 75237 (469) 608-6228
Super Wally 9301 Forest Ln Dallas TX 75243 (972) 437-9146
Super Wally 13739 N. Central Expy Dallas TX 75243 (972) 656-2501
Super Wally 4122 LBJ Fwy Dallas TX 75244 NOP (972) 980-2195
Super Wally 15220 Montfort Dr Dallas TX 75248 NOP (972) 233-0438
Super Wally 18121 Marsh Ln Dallas TX 75287 (972) 307-6978
About leaving your RV unattended at the store.....I have never done that and I would have to make that decision based on what I thought of the stores location once I arrived.