Thanks for all of the replies and suggestions that everyone has posted.
So after a few meetings with our traveling companions we have currently decided on the following itinerary which to subject to change:)
We have decided to fly into Paris instead of Frankfurt.
We will stay three nights in Paris. The first night will be a do nothing as we don't currently know when we will arrive.
We would then travel by train to Frankfurt to pick up the rental car.
We will then travel towards Arnstadt where quite a few of my wife's ancestries are from. We will spend two nights in that area
From there we would travel to the black forest for 2 nights.
After the Black Forest we will travel to Munich again for two nights
From Munich we will travel to Schwangua for three nights. We are planning on staying at Hotel Muller with the castle package.
During the three nights we will travel to Garmisch for day trip
From Schwangua we would travel to Salzburg stopping in Innsbruck. We will stay three nights in Salzburg.
In Salzburg will drop off the rental and travel to Budapest via train.
We will board the Viking Cruise ship there and travel to Amsterdam.
While in Budapest we will travel to Napkor to check on some ancestors of our traveling companions.
The plan for the last week after we get of the boat on Amsterdam is a little open yet. We are looking at Antwerp, Brussels, and Brugge.

Even tough ew haven't went yet we are already thinking about a return trip but traveling via RV. We currently travel in an RV in the US so we are sorta used to that type of travel.

The plan up to and including Munich is so amazingly bad and rushed ... I really give up.

RV in Europe is a bad idea. Especially if you want to visit cities.

RV can also be a great way to tour around if not going largely to large cities - like the South of France - nice camps everywhere and even for large cities there are camps on edges with public transit into towns. So not always bad - but depends on what you want to do. rentals also costly and gas guzzlers too. Camp fees however much lower than hotels but still not that cheap. July and August camps in popular places may be full so have to book ahead with an RV.