Many towns will have internet cafés and some will have access through their public library.

I don't think you'll find a pan European wifi mobile access solution. But if you Google "wifi in european campgrounds" you'll find a lot of place you'll be able to get WIFI. Coupled with Michael's suggestion you may find this is enough connectivity.

And what about a internet stick? Are there europewide plans?

I do not know if there are europewide plans, the trouble with cellular services is that they are relatively cheap in their home country, but as soon as you begin roaming (that is, using a SIM outside its home country) prices go from expensive to extortionate. For example - I have an Italian SIM card in an Ipad; 2GB a month for 9 euros. And when the gigabytes are over, it simply goes slower but the charge does not change. - If you want to use it in Europe, there is a special offer at 6 euro per day, 150 MB per day; but if you forget to activate the offer, the ordinary price would be something like 800 euro per gigabyte (!). - So sticks are a possibility, but you would have to carefully understand what your are doing, and probably the best thing to do would be getting a new SIM in each country - highly impratical.

Thanks for these infos. I guess, since we will stay not a long time in some countries, we could look for free wifi there and get sim cards when we stay longer. I think this seems reasonable.

I've found this where they offer 100mb for 2€ with europaSim, would that work?

http://www.motorhomewifi.com/product/europa-sim-unlocked-huawei-3g4g-dongle/

Yes, that is a pretty good price for a roaming day pass.
The system of prepaid roaming day passes is the usual option you get from mobile ISPs in Europe, asps described another above.
A similar plan from T-Mobile costs €5 per 24hrs with 50MB included.

The only downside may be that the plan / dongle you found has no cap that deactivates the access once you exceed the limit but lets you run up extra costs. Therefore you need to check the MBs you use regularly (usually done via a website of these providers which monitors your data usage)

100mb is quite okay if you don't use any streaming video or audio. Or any other app that uses lots of data, e.g. navigation/ maps. Or limit that to times you have access to WiFi.
But it's fine to check e-mails (deactivate automatic download of attachments, if applicable), surf the web, and check Facebook.

Re. coverage you may want to check if Croatia is included as they are not in the EU.
And if "per day" means per calendar day or per 24 hrs.

It looks like europasim is an Italian Vodafone card. Its fee plan looks very similar to the sim I quoted here above; basically all providers offer such plans with some variations in the costs and allowed data transfers. Sometimes even understanding the fee structure may be difficult, the important thing would be not overrunning daily limits.