I had the same question working on the front-drive gear box on an EKS110 order picker. I finally just pulled the motor out of the gear box, really quick and easy, just take notes, pictures, ect. on everything you unplug or remove.

if forced to guess the why, they ( ze germans) assume a. the oil will last longer than a customer will lease the unit for; b. no customer would choose to service it themselves when they could send the unit to a fully trained and authorized Jungheinrich service facility...

So they didn't think it would be worth it to put a fill-hole in the gearbox?

¯\_(?)_/¯

Hope your able to figure something out, good luck.