Road Trip Germany

At 2 to 3 hours driving per day, if you go directly to Amsterdam it will take you 3 days. And 3 days back.

That's 6 of your 10 days. How long do you want to be in and around Amsterdam?

i dont want to go directly there but rather want to take it 4 days there, 2 days to stay and 4 back. i want to stop at nice nature places like Goslar

There will be approximately 8 hours of daylight at that time of year - and weather may well be overcast if not rainy, or something else. Just something to consider when driving an unfamiliar car/rv in a foreign country on local roads near to nature.

You'll pardon me asking, if you don't want to do anything in Berlin when you arrive or when you leave, why are you going to Berlin?

I have only rented an auto (sedan) in Germany, but I would think you have to research legal overnight stops for RV's. Germany is more permissive than the US, but perhaps more strongly committed to residents' personal privacy. You're not going to find a Walmart parking lot to sleep in.

As well as the issue of finding somewhere to park to sleep every night (most sites will be closed in January), the weather and the short days, you will not be able to drive into the centre of Amsterdam. You will need to find paid parking outside Amsterdam and get public transport in. The same will apply to most big towns.
For example, the centre of Goslar is pedestrians only (like most towns). Many car parks, including most under-cover parking, has a height limit of 1m85 or 2 metres, too low for a camper van.
Click here for Google Streetview of Goslar centre - you can't drive there, deliveries only.

Sorry to be a wet blanket.

January is winter. In winter, we get snow. Snow is a real issue in driving. Do you really want to go to Germany, in January, and drive in poor weather? It's true that Germany does not get a huge amount of snow, but it may get some.

So, to make your planning more sensible, you need to decide where you want to go. Nigel has suggested that Amsterdam is not a good idea due to distance. There are places just south of Berlin which are canal-based - the Spreewald. Not sure how much is frozen in January but considering closer stops makes sense.

What in particular has suggested Amsterdam?

I lived in Augsburg, Germany for four years and traveled extensively from there. We usually drove the family car, sometimes we took the train, sometimes we found a good bus tour.

We never did RV camping. If you plan to do this trip with an RV, I advise you to do research prior to going. RV travel happens in Europe, but no where near as in the USA. I did notice more RV travel in Great Britain on our road trips there.

No way would I want to travel in an RV in January in Germany. If you get snow, you will have to negotiate highways that may have ice. The Germans are great at trying to keep the highways clean of ice and snow, but it can be a problem. Also, camping areas may not be cleared.

It gets dark in Germany in January at 4:30pm. Days are short. It will be cold. Plan on cooking outside, that is another issue.

My Daughter and her family did a camping trip with an RV in France, in the SUMMER and they enjoyed it.

Finally, I doubt if you will be able to camp close to the inner cities that you may visit. Amsterdam is great, but when I visited there, even having a car was bad enough. Finding accommodations in the inner city that provided parking was just not possible (perhaps at a five star hotel away from the city center. You see more bicycles in Amsterdam than cars.