national parks in Europe are free for everyone (honestly I had no idea that in the US you have to pay to visit NATIONAL park)... (but it's really stupid to waste time in Europe with national parks when the historical sites are incomparable to anything in the US - ignore nature altogether and just visit castles, chateaus, cathedrals, historical centers of cities etc)
Cost of gas varies a lot from country to country, if I were to estimate average for whole west and central Europe, I'd say about $1.5 per liter
You don't give your age, but to rent an RV anywhere in Europe the driver must be at least 23 due to insurance requirements, and 25 with many companies.
You also need to know that renting any vehicle in one European country and driving through other countries and, presumably, leaving it in a country other than the one you hired it in is an extremely expensive prospect. To get an idea of prices and requirements just look at a few hire car companies online.
I take it that you do not intend to visit any cities while in Europe, at least I hope not because you will have trouble finding parking for a RV and driving will be a nightmare. RV parks are called caravan parks, or campings, and in summer can be very crowded and require early booking. These are also generally located well away from cities.
An RV? In European cities with narrow streets? Are you insane? Forget it!
Much cheaper and easier to stay in hostels and use the trains.
Are you a Member of AAA. You can use their travel service to get information from the German Auto club.
http://www.adac.de/produkte/autovermietu...
http://www.adacreisen.de/campingwelten.h...
With more than 18 million members in May 2012, it is the largest automobile club in Europe
The ADAC has reciprocal arrangements with a range of international affiliates, including
The AAA in the USA.
http://www.bfn.de/0308_nlp+M5054de7a952....
http://www.bfn.de/0308_np+M52087573ab0.html
YOu have no clue, do you?
