OKay, so I just returned from our third summer in a row at Yellowstone. I'm sad it is over and am recovering by lurking in the forum and strategizing my 2015 trip, which is already on the books.
This summer I felt like we did our best on prepping our "picnic basket" supplies and wanted to give advice on lessons learned an
1. The lunch meat they sell in the park, is not what I would normally purchase, and we chose not to consume ie Oscar Meyer Bologna, etc.
2. Fruit is scarce in the park: Roosevent and Tower stores do not have fresh fruit and we spent 6 nights in this section of the park.
3. Packaged foods such as chips, cookies, bread are available.
Outside the park (Smith's in Jackson and the small grocery stores in West Yellowstone) we purchased our fresh fruit and sticks of salami that we could use for sandwhiches. Next year, we will bring dried fruit to supplement the end of the trip, when our fresh fruit isn't holding up and we are no longer venturing out of the park.
I hope this helps and would love to hear other strategies. Oh, and the wine selection in the section of the park we were in was pretty sad, better to buy outside the park.