I think it's a question of application, scale and time...this quote HERE
from the British Stainless Steel Association...
Galvanised steel in contact with stainless steels is not normally considered to be a serious corrosion risk, except possibly in severe (marine type) environments
...suggests it's a problem, possibly, in severe
marine environments. But, I don't think dipping a trailer in every week/month or so is 'severe'. When we pull the trailer out with the boat on, the trailer gets hosed down too. To date, after three years, there's minor cosmetic rust on a few of the mild steel bolts/nuts/washers, none on the stainless fixings. The exception to this is the roller washers - zinc plated steel - some of which were bright rust and the rest which look like they were made yesterday. This to me suggests more of a plating process issue than anything else.