You asked “if a step van could be built turnkey for $20,000 but the bus would cost $40,000, would the attention the bus attracts (increased sales) be worth the higher cost and risk?”
My first thought is hell yes, but I would most likely not use it as an event mobile food vending unit but instead use it as a upscale mobile restaurant. If you must do short tours, do them of botanical gardens, historic neighborhoods, vineyards, historic sites, the list of possible tours is endless. And I’d serve top of the line  food, with a great chef. If you choose to go to the other end of the spectrum, you could do pub crawls, and special event dinners. and change the menu according to the event.
1 A holloween party tour.
2 A Christmas Party (depending on weather) or a Christmas shopping tour. (Yes it’ll be cold but people will dress for the weather)
3 Ice skatting party
4 Ice fishing event.
5 Spring garden tour
6 political fund raising event
7 chilli cookoff event
8 BBQ cookoff event
At any event that involves alcohol customers are reluctant to drive  so you become the limo and the restaurant all in one.
My list is short and I apologize for that but I just finished a long day and we killed them at the annual St Pats Parade. Just 3 people  two employees and myself  served more customers than any restaurant in Springfield serves any Saturday night on average. We worked out tails off. but it was well worth it. Go for it if this thing is structurally and mechanically sound you’ll have a win win situation.
good luck