Drove a cs camper every day for seven years and would say the following:
1) the temperature guage is exactly that ...a guage, not a scientific instrument. Some read high, some read low ..comparing needle locations between vans rarely gives a meaningful result.
2) nonetheless the cs needs to be driven by temp guage rather than speedo, especially on the motorway. There is a sweet spot somewhere just above truck speed but still below 60 mph where it will happily trundle along with the temp needle just showing warm, not hot. Any prolonged speed increase will send the needle upwards. This is your signal to take it easy, get off the gas, sit back and relax and just accept the fact that your journey might take an extra few minutes
3) if you need to speed up / push it up a hill and get the temp soaring, make sure that you slow down afterwards/utilise the downhill bit to get the temp down again
The cs simply is no motorway cruiser. It was slow already when it came out and almost 30 years later it certainly isn't able to keep up with modern (motorway) traffic flow anymore.
Accept that, stick with the (slow) trucks and be happy.