I live in Wyoming and have toured often in Yellowstone.
I agree with Seeker333 - there is no problem eating in Yellowstone.
Most of the lodges have very pleasant dining rooms.
The one at Yellowstone Lake Lodge has a stunning view.

Make sure to eat on the deck at Dornan's in Moose at the south entrance to Grand Teton NP.
It has the best view of any restaurant deck in the world.

PS - There are campstores near most of the larger campgrounds in Yellowstone, too.
(Grant, Bridge Bay, Canyon, Tower, Mammoth)
(There are NO campstores near Norris and Madison.)