Yes, as long as you're fairly level- putting slides out is fine. You can also drop landing gear and stabilizers if you need- just be careful that you don't poke holes in the blacktop if it's hot out.
Make sure that you stay in a row of spots and don't take up extras with your slides. Also make sure the slides don't extend into the roadway and impede traffic through the parking lot.
My fifth wheel is 44' long, my truck is 22' long. Bumper to bumper, we're about 60'. I take up the same number of spaces (a lot) if my slides are in or if they're out.
The only people who really care are folks on Internet forums.
These match the "Code of Conduct" published by Escapees (which is agreed upon by quite a few RVing organizations). They aren't rules, just strong suggestions:
https://www.escapees.com/wp-content/uploads/2018/03/88gnpnd17.pdfYou can read more about Wallydocking in this article:
http://learntorv.com/wallydocking