I didnīt have to Google poly although (thanks to the late hour) polynesian first came to mind and then I came to my senses.
I think that the biggest problem you will be up against in many German hotels is how many people a hotel will permit in a room, period. This has nothing to do with paying for additional people - itīs just a question of hotel policy.
A question to the OP: how has this worked out for you elsewhere in terms of finding the right sized bed?