28 is a bunch to feed on a gravel bar...Here's a couple cheap quick meals that work well with a big group..Pulled Pork...smoke it ahead of time, pull & freeze in 2 gallon ziplocks...reheat in an aluminum pan with some liquid & bbq sauce...Soup or Chili...make ahead, freeze in 2 gallon zip locks, reheat. Bring a big stock pot...after day 2 you will probably be limited to pasta, rice, oatmeal, dehydrated meals and add water type stuff unless you catch some fish to eat. If your bringing fresh meat...vac pack it so it wont get soggy in your cooler..Separate cooler for food and beverages. Block Ice and drain it if you want to keep ice alive longer. Have fun...sounds like a blast.