So true, when I had my pizza truck we sold bottled water and Mexican Coke in bottles with real sugar not high fructose corn syrup. San Pellagrino cans of flavored water and the biggest issue was trash! Carry lots of heavy duty bags! Most trucks don't have enough disposable/recyclable capacity. People buy food from you, walk a block down the street, discard their waste on the ground because there are no trash cans or they are full, and now you have negative advertising because your branding is all over the street!
No one I know in the mobile food truck world has a soda fountain. Sometimes, like you, I think, "That's my edge" then I have to listen to the voice of reason, AKA my Wife...."Why do you think nobody else does it?" Duh cause it doesn't work for a mobile vendor!
Good luck, it's a hard luck life.