I was thinking...should I apply the decals over the winter white wash? Did germans paint temp markings on vehicles or not? Maybe I could make the decals peek through the old wash or something.
What do you think, any ideas?

I have seen many pictures of whitewashed vehicles showing the original tactical markings.
i believe they masked it off, somehow to prevent covering it. (probably depends on how wash was applied.) Typically you can see the basecoat of paint behind the tactical marking.

I have seen tactical numbers painted over whitewash and I have also seen the white wash painted around the numbers and insignias, with the base coat showing thru...

...And I have seen new numbers painted over winter camo with the original numbers still visible through the white paint on the same vehicle. Needless to say, the new numbers were of a different size and were not in the same position as the original.

I have seen mostly they painted around the crosses and IDs.