Simon,
You may want to contact Hobby in Germany, or the UK Hobby agents, or the UK owners club, for the manufacturers original tyre specification because both sets of tyres just sound wrong.
It is normal practise to fit standard car or light commercial 80 profile tyres to a caravan.
I can't remember a manufacturer fitting ultra-low profile (55 or 65 ratio) tyres to a caravan? I'm guessing they would be closer to a 165/80 x 15 or 175/80 x 15.
The side wall of the 80 ratio tyre makes up a considerable percentage of the caravan suspension and shock adsorption - low profile tyres transmit a lot more of the bumps into the van - and cornering forces aren't a problem with a caravan.
And you probably need to look at the tyre pressures.
Robert