Hi, OP here,

Many thanks for the information - but not really what I was hoping to learn

My understanding is:
Some campsites do not actually have places for tents - so the only option here is to pay for an RV place?
Hiker/Biker sites are for tents and cheaper than the RV option?
Campsites will refuse you if they are "full" and send you on your way even if there are no other options in the area?

So my next question is.....
For those recommending reservations in advance, how do you do that other than for a short tour?
Realistically, how do you know what date you will be at x campsite when that is 2 months after you start?
I can't imagine myself even booking something 2 weeks down the road - there's just too many possible variations that can occur in those 14 days.

@ Rowan - in my experience most campsites have long-term residents (that covers Germany, France, Belgium, NL, Austria, Italy, Spain), but only once did I have an "iffy" feeling in one - but that was wintertime in NL, and most of the residents were migrant workers. Having said that, when I was fixing my travelling partner's flat tyre the next morning in the rain, I had no shortage of offers of assistance. There is a different vibe in these places, but nothing that has put me off. It's been German speaking campsites that have squeezed me in somewhere when I was late and it was full.

Thanks all!

Frank