300 miles from midday is quite ambitious! However, we've stayed at Camping Ohmbachsee using a similar route. It's a Camping Cheques site (378) and you can get full details from their webite. It's little bit out of the way, but it's a decent site. If you don't quite make Saarlouis, then I can also recommend Camping Mamer which is just off the motorway in Luxembourg (around 230 miles) and is an excellent transit site and well used by the Dutch. Doesn't have its own website, but details here www.eurocampings.co.uk/en/europe/luxembourg/campsite-mamer-105331/