I might add here that at commercial establishments we don't want to look like we're camping (local neighbors sometimes object) which is a secondary reason for not putting out slides, etc. I was going to suggest not unhooking but didn't know if that was a reasonable suggestion (thanks, Lowell!) because we're motorhomers. In your situation I would want to look as temporary as possible which means I would not unhook unless I needed to drive somewhere else such as to visit a friend.
Men might feel less threatened in some situations than a woman. Before settling into a place like Wal Mart, look it over carefully. First, you want to check for signs that say No Overnight Parking. If you see those you want to move on and park elsewhere. Second, look at the lighting and stay in one of the well-lit sections of the parking lot (taking into account you also want to be out at the edges where you'll be out of the way). Third, look at the clientele. If you see a lot of young people (sorry, kids) who just seem to hanging out, do they look like they might cause problems? If so, move on. If it seems they'll leave and the place will quiet down then you'll probably be okay. In any case, I would close my curtains for privacy. You don't need to have anyone looking inside and realizing you are a woman traveling alone with a son (is he a child or older so looks more able to defend himself?).
Most highway rest areas do not allow overnight parking and the reasons are first to leave enough room for people who need to stop briefly to use the facilities and second because there have been serious issues at some rest areas which you would not know about because you don't live nearby. Many years ago we overnighted in a lot of places (including rest areas which were permissible in the 1960s) but as we've learned more about them, we've gravitated more to established campgrounds. They don't have to be fancy but they may be safer and you might feel more at ease to get out and walk around.
It's great that you and your son are able to this and that you can meet your husband along the way.
ArdraF