Your DIL is usually home when you visit so you expect her to cook, sort your drinks because you won’t help yourself out of your own discomfort?
Do they have enough money? Can they afford to offer food and drink?
Go and find a cafe and bring a coffee/tea and food just for yourself in with you.
DS, if I prepay 100 for food right now, could I please be included in meals and cups of tea etc when I next visit? I really would enjoy a cup of tea and a buscuit when the kids are eating.
These comments can't be serious ?
Are you seriously trying to justify this batshit behaviour by the OP's son and DiL?
Would anyone whose Mother / MiL, or indeed any one not related was coming to see you, on a pre-arranged visit not offer them a cuppa when they arrived, after driving a couple of hours or more to get there ? 
Yes, we can all ask why it is only the DiL and not the DS that the op is blaming, but the behaviour, if as described, is completely out of order and downright rude.
The only question is why the OP didn't speak out in the first place, and possibly why she would go back.