We always pull the flush lever slightly to fill the bowl to around a 3rd before getting started giving them something to bob about in same as a domestic loo then flush the lot, no marks, don't know if this would use too much of a euro vans cassette allocation but with 50 gallon capacity we only have to drop the kids off at the big pool once a week
I believe the RV bowls with drop to holding tank are more user friendly
I have a quite deep porcelain bowl, put an inch or two of water first then do your duty, flush and job done.
The euro cassette thingies are a big different methinks