The chemical process of passivating doesn't happen that
quickly. That's why, after you wash with vinegar, you wash again with warm soapy water, to remove any vinegar residue before it has a chance to harm the surface.
In addition, you're using only 5% acetic acid, not 100%.
If you're worried about it, you can always try it on an inconspicuous surface (the belly pan?) first before trying it on the whole trailer. Or keep looking for other alternatives to kill unwanted biological growth on your trailer.
Or leave the biological growth alone and turn your trailer into a chia pet!