Unfortunately, that food truck operator is a victim too. They live and die by reviews, and if everyone else does it, they have to too to survive. The whole thing is jacked up.
I imagine that food truck operator would do a lot better by simply asking "Like or hate our food? Leave an honest review on <site> for a discount"
If their food and service is good, they'll get the positive reviews they need
I.e. your plan is doomed to fail.
Except maybe if you include a line like: "were not going to ask you for a 5 star review like others do, but reviews really make it break us and we're trying to be honest. Thanks for the support"
The only acceptable way to do it is "Please give us a review at [site]". No discounts or promotions.