Compared to your British curry houses, many German Indian restaurants don't stand a chance. There are exceptions, though.
Expect better restaurants in the bigger cities. "Curry houses" in small towns are not really Indian when they serve "curries, pasta, pizza, steak, sauerkraut and crepes".
Your voyage down the Rhine will most likely not let you step out in the Munich area, but if you happen to come across the Munich airport in the city Freising, there is my favorite curry house in Germany: New Indian Palace,
http://www.indianpalacefreising.deIt's the one that inspired me to cook my own since I'm no longer in that region ;-)
Good luck and have a nice trip!