Start by clarifying for what are potatoes used for. I'm assuming it's only for fries (but don't exactly know the full menu of McDonalds in the US).
I would divide this problem into two components: fries actually consumed by customers + fries that are wasted in the process (as a McDonalds customer I would say this is a really relevant variable for them: everytime they make a mistake with my burger...I always ask for a package of new and warm fries!).
You can estimate the first one by using a top-down approach:
+ Population of the US
+ divide into age groups, assuming a uniform distribution across them
+ get an estimate of the average meal consumption per week and escalate per year for each group (children and teenagers consume more I guess)
+ assume that every meal includes one package of fries (some include more for sure; others none) and weight them (100g per package?!)
On top of it, I would assume there is something like a 2-5% waste rate.