We stayed overnight at Stellplatz in Fussen and parked MH in Castle car park to visit both castles, and Stellplatz in Cologne, brilliantly situated right on riverside cycle path straight into centre of town with beer gardens along the way! Both in Bord Atlas book.
We stayed at the Stellplatz a few miles short of Fussen at Camping Bandwaldsee on the lake. As you turn into the site turn right and there are 24 stellplatz pitches. On the left is the main camp site and where you pay. You can use all the camp facilities including the showers and the excellent restaurant for the price of the stellplatz! There is a bus stop outside the site to Fussen which goes via the castles. Also a cable car is close by and worth going to to the top where there is a lovely bar/cafe/restaurant and they do paragliding off the top of the mountain!
Can't help with a stellplatz at Cologne as we did not stay there but did visit. You must go to the Lindt chocolate museum which of course makes chocolate on site!