Hello,
I recently had a stand heater from Eberspächer (the same one as you) installed.
The installer told me that the only permissible installation orientation is vertically, so that the water hose connections point downwards.
If necessary, you can also call a Bosch service center near you; they install Eberspächer auxiliary heaters and should be able to help you.
http://www.bosch-service.de/
Best regards,
Jürgen