I take it numbers from Atomic Rocket aren't good enough?
It gave the following numbers in kg/Liter. Since both liter and m^3 are volume measurements, you just multiply to convert. There are 1000 liters in one cubic meter.
Frozen meat and veggies : 375 Kg/m^3. "fresh" foods, 250. Dry/canned goods, about 500.
From handling MREs, I know from personal experience that dry foods are not that dense. Even if you removed the air from the MRE package, it will not have that much density. Another factor here is that I don't think it matters that much. Since volume is cubed, I don't think that current missions have to optimize for volumetric density so much as reducing the mass. If you make the payload section of your rocket slightly longer, it will only have minimal effects on the air resistance. (while, for a given rocket design, increasing the payload mass comes up against hard limits)
So I would pick the lowest of these numbers. Imagine a food storage pantry filled with stacks of compact MREs, with extra space around them so that kerbonauts can get to supplies at the bottom of the pantry.