Fast food chains such as McDonolds and Burger Kings always have a no-pay restroom (loo) and something to drink that won't get you arrested.
Truck stops usually have a slightly better selection of food and free restrooms, but are more of a problem geting safely in and out.
I have to stop for gas every couple of hours anyway, but I try to find one with a fastfood place nearby. Used to be that they had free restrooms and often free coffee for customers, but they also used to check your oil snd tires too. Now, it is common to have to dump 50 cents or a dollar into the slot to unlock the door. They don't change your money, so if you don't have the right coins, you are litterally SOL.
Rest areas are a good place to stop and stretch out the kinks, but watch out for idiot drivers, avoid the vending machines like the plague and only use the facilities in an emergency.
Especially in the western half of the US, be extra careful around the good old roadside bushes, you are likely to be sharing these "facilities" with a rattlesnake.
