I can't speak for our German brethren, but I would think that either version is acceptable, "Unimog(s)", based on precedent within the animal world.
For examples: there is one Buffalo and/or Water Buffalo; and there are multiple Buffalo; similarly, there is one Deer, Cougar, Coyote, Bear and in multiples of those animals there is no 's' necessarily added.
However, with plants, one tomato becomes tomatoes, banana becomes bananas, etc; even though one corn remains multiples of corn, and bamboo remains bamboo.
So, either is acceptable, depending upon 'local' custom.
An inanimate example would be "ship" becoming "ships", but "aircraft" remains "aircraft" when plural.