While a RV has a bed, it is usually already occupied by a man and a woman so not prime territory. In ten years and probably 15 times using a truck stop (arriving late and leaving early) I have never seen single women roaming around. I rarely go inside and have never had any problems. I do get out and do a walk around before taking off, but I do that everywhere, not just truck stops or rest areas. I've probably stayed in rest areas 10 times in ten years without any problems too.
I have found Sam's Clubs a quieter option than Wall-Mart but I usually arrive after it has closed and leave long before it opens. Did this in Valdoster, GA last week where there were probably 20 RVs at the W-M and I was the only one next door at Sam's.