At this time, it is usually not enforced, with some very local exceptions - e. g. Stuttgart.
With a truck with US registrations, I would not really worry. Unless you get stopped for another reason and the police really really want to find something, I do not think it will be an issue for you.
That is Germany, 2019. Things may be different two years from now, and in other countries.
We are supposed to be getting toll stickers here. Their price will depend on the environmental impact of the vehicle. Many other countries have similar systems in place already (I wonder how many countries you can visit before you run out of sticker space in your windshield).
I have no idea how they are going to classify your truck. They may not even know themselves... Sell you the most expensive class? Maybe.
Long distance highways ("Autobahn") will be fun in some of the countries, anyway. The Austrians require something called a "Go Box" when you are over 3500 GVWR. Other countries do similar, funny things. Europeans like to bicker about the EU and how it takes their freedom away by making EU-wide legislation. Well, the highway toll systems are not harmonized so far, and I really wish they were. Every country gets do do it their own way...
