For what it's worth, there's over 200 Gourmet foodtrucks in LA and OC, and I am not aware of any that sell fountain drinks. Too many equipment/space/logistic issues.
Drinks account for 17% of gross sales for us. A lot of stops, folks just buy food, because their employer has free drinks in the break room. Bottled water is about 40% of that. The balance of what we sell is somewhat different - Glass bottle Coke, Sprite, and Fanta Orange. We sell bottled Virgil's or IBC Root Beer, and Hubert's Lemonades. Glass bottles sell better than cans out here. We get a buck for waters an $2.50 to $3 for bottled drinks. We tap into Hubert's and Virgil's social media to publicize our stops too.
Iced Tea (Counter space for urn, Cup, scooping ice, putting top on) all take time at the window, and over 4 hours, can cost you sales. We upsell to combos, or add fries, and have never worried about the margin of an Iced Tea item.
Instead of a cup with advertising, look at postcards (instead of business cards) for the truck. We have them sized to fit our napkin dispenser by the order window, We can get all our info on them, plus pictures of truck and our product. They have legs vs. a throwaway cup.