There are not many camping sites, but there are some in Germany and some in the Netherlands. I would assume that neither of them have big problems in English, but if you want to make sure, you might want to pick one of these two:
Then there are some in Germany, such as
and one more in the Netherlands:
Of course, all of these are not for free.
Generally, you can park anywhere with a camping car where you could also stop with a normal car and sleep there - just like anyone can sleep in their car to take a rest from driving. What is not legal is to take out a table and chairs etc and have a living-there kind of setup. Also, you might want to change the place of the car after 1 night, since a longer stay might be considered a permanent establishment of sorts and is also not allowed. You also have to watch out for blue parking signs with an additional white sign that might restrict certain cars types from parking there.