I take my RV through Flag several times a year, fill up on gas, get some lunch, etc. I've never been hassled what-so-ever. I am considerate where I park so I don't take up a lot of space near the front of a business.
You are not allowed to overnight in a parking lot in Flag which might be where you've heard they are not friendly. However, you can camp in the National Forest just outside of town.
Send me a PM if you need a suggestion for a place to boondock between Flag and the Grand Canyon.