You won't need a ferry (other than perhaps across the Rhine or some obscure German river) between Rotterdam and Berlin. They are cheap, no discount required. Trust me.
Great roads to ride, mate? Leave Rotterdam, head due east, arrive Berlin. Seriously, have you tried looking at anything at all vis-a-vis a map? Have you tried looking at the sticky which some kind hearted bod created with guides to Germany?
http://www.ukgser.com/forums/showthread.php/416452-Germany-calling
Come on, you've decided to go to Berlin for some reason or another in the spring of 2018, you can make a bit more of an effort, surely? But hey, let's not be too harsh, here's some ideas to get you going:
1. You want to do the journey out to Berlin and the return back in three days each way. You could choose any route you like, then divide the distance into three equal thirds, then find yourself two overnight hotel stops at or near the point where the two one third distance lines end. Alternatively, you could ride out say 50% of the distance, stop for two nights and use the second day to ride out on a circular route back to your hotel. From the links below, you can see that Goslar might well make a good two night stop. Then find yourself ride out routes involving Goslar. I Googled two words: 'Motorcyle Goslar' which brought up:
https://www.goslar.de/presse/presse...service/603-stadtforst-weiter-auf-erfolgskurs
Which brought me to:
https://www.goslar.de/contact-servi...ng-to-goslar-special-routes-for-motorcyclists
You'll need your map to work out quite where they recommend riding but it wouldn't be too hard to do.
And for simple hotel (biker friendly, so they say) to:
https://www.goslar.de/hotels-apartments
2. I then added a third word to my Google search involving Goslar, 'motorcycle routes Goslar'. This turned up lots of ideas.
3. Thinking outside the box a bit, I tried three other words in Google (fairly sure I wouldn't be the first bod to go from Holland's major port to the capital of Germany) 'Rotterdam to Berlin' just to see what popped up or if there were ideas I could steal. Bingo! You could turn this coach trip around a bit, perhaps. Reading between - or even along - the lines you'll be able to work out what the tour involves, the places they'll go (if not the specific roads taken) the towns they'll visit, some hotel ideas (see also Booking.com, Trivago, TripAdvisor etc) all enough to get you going.
http://andersonscoaches.co.uk/berlin-the-harz-mountains/5312691
Or this:
http://www.mancass.com/index.php/tours/110-berlin-the-hartz-mountains
Or:
http://www.glentonholidays.com/itineraries/9021-harz-mountains-and-berlin
To get you in the mood, here's Leonard to get you going with its Euro-bop inserts:
<iframe width="560" height="315" src="https://www.youtube.com/embed/ytwte4-9Gec" frameborder="0" allowfullscreen></iframe>
Come on YOU can do it. Give it a go. You found your way to UKGSer, so play about on the web. It's free, you can't break it and you know yourself better than any of us. You've got perhaps seven months until spring 2018, so there's lots of time. Start to put some ideas and good websites you find up in this thread. Who knows, it could well become the 'Go to' thread for the whole worldwide web; wouldn't that be great?
Richard
PS There are several threads about North Sea ferries, including discount codes. Failing turning any up, contact Micky when he's back from touring the Highlands and Islands.