To me it is unacceptable behaviour. I was brought up to have manners and respect for others and for rules, I wouldn't have dared do that growing up, even now I wouldn't help myself to things from the fridge or rummage through cupboards or go into anyone's bedroom without being told I could, not that I'd want to raid or rummage anyway!
I wonder then if DIL has a more relaxed upbringing and doesn't realise it is wrong, or does and doesn't know how to be a stricter parent, both could leave her sensitive to reproach, not that that is an excuse. Maybe some subtle hints along the way if you are wary of tackling them outright and not seeing them for a while.
I would be tempted to make a point about the
Site Rules and how issues like noise upsetting the other residents could affect her ability to stay there or live there harmoniously at the very least. If they are there I would lock the door (how are they getting out and back in?) and if they mention wanting to go out then I'd bring up with the parents about the rules, you'd hope it would at least shame them a bit!
If you can't come to some agreement about meals, if there is no specific dietary reason for any "special" food then I would stop inviting them round for meals.
Is there a lock that means you can secure your bedroom door from the outside? I know you shouldn't have to but it's a private space and they have no right to be in there unless invited.
Fridge and cupboards are more difficult. Is it only certain things they will take you could stash elsewhere e.g. biscuits in high cupboards. If they ask for something just tell them you don't have it and "jovially" mention you are a pensioner so have had to cut down on treats. Fridge stuff is trickier still though. I'd be tempted to buy some "thought-provoking" fridge magnets such as My House My Rules, If you want it ask etc... :)