I think tying this resting system to how much food/supplies you have is the easy way out of something that could be more immersive and enjoyable.
Forget the food...It would work better is this was tied to creatures spawning during rests:
On story/easy mode just let everyone rest whenever if they have whatever food...even no food for just story mode.
For other difficulties, introduce a creature spawning table dependent on level, time of day (day night cycles please
....) and the area you are in, and roll a check. So you get a nice risk/reward strategy for wanting to rest, not just a <<just get supplies>> game-play. So at the first rest 10% chance for creature xxx to showup, 5% lucky roll (bellow). Then another rest soon after, up that to 35%/8% more difficult enemies/interesting rewards etc...
The more you rest in that area, the higher the percentage of creature spawning gets (this tied to the difficulty). And maybe during a VERY lucky roll, no creatures but some kind of allie/friendly spawns for items/bonuses or just fun story elements. Like a wandering bard with xxx story leading to a quest line...a hamster with a story to tell...a <magical><cursed> item appearing out of nowhere which is actually a gate to an Inn in planescape's Sigil city of doors...
This is actually kind of close to what BG2 also did (with the SCS tactical encounters and other mods).
