I've visited the parks multiple times per year since 2009. I agree with CloudDriver, and would stock the larder in Billings. Supplies of every kind are more expensive both inside the parks and in the gateway communities. Especially produce and gasoline. The price per banana in the park last year was 79 cents! The parks are huge, and require a lot of driving to enjoy. Keep in mind that the top speed in YNP is 45 mph with many spots of lower speed limits. Plus, the inevitable traffic jams for wildlife sightings will occur, and there are spots of road construction as well. The gas is much more expensive within YNP, but you will need to purchase while inside the park. I would budget for that. Alcohol prices within the park are not bad compared to outside.. so waiting to get some in the park won't break the bank. At Flagg ranch, you will still be very far from a reasonably priced grocery. The closest will be on the south side of Jackson at a Smith's like mentioned previously. This will take the better part of 1.25 hours one way to get to. If you're on a grocery run, take the quicker path around the Tetons outer loop that will take you through the Moran entrance in order to remain on the HWY.
Finally, a friendly reminder that YNP requires barbless hooks for fishing anywhere in the park, and you will need a park permit to fish.
I have the Rand McNally app for the iPad. I cannot recommend this. It sent me through a busy business/strip mall in Tuscon only to send me to a NO LEFT TURN intersection to turn left at. Going past the intersection left me no opportunities to U-Turn and to smaller and smaller roads. I ended up unhooking the towed and turning around in a small housing community. Then I find out that there was a direct RV-friendly route straight to my CG from the HWY that I was on originally if we would have stayed on it 8 more miles. The app also thinks I am "off-roading" it quite a bit during a highway trek. If I have to use a mapping unit plus a GPS... the GPS is not worth it to me. These aren't isolated events. All too often it sent me on routes that aren't RV-friendly and I feel that I need to always back up this app with other mapping programs in order to verify that we won't be stranded without options.