Killing the unit in the barb camp doesn't activate the German UA. You have to actually move a unit into the empty camp. With a melee unit, you always advance into the hex of the unit you destroyed if you attacked. With a melee unit, if the barb unit attacks you from the camp - or you kill it with a ranged unit - you have to actually advance into the camp to activate the "conversion".
ANY UNIT can move into the barb camp and then you either get the gold or a barb unit plus the gold. The only thing that matters is that a German unit occupied the camp (even a worker, GG, settler will do). The camp disappears as soon as it occupied so you only get one chance.
And if the camp is empty, the nearest barb unit will move as quickly as possible to occupy the camp OR, if its the right turn, the camp will spawn a new unit which will occupy the camp.
BTW, thanks for the information on the Otto UA - I haven't played them yet but was going to try them next. I would have gone about killing all the enemy naval units trying to covert them rather than park next them. It would just never have occurred to me to try it that way.