I have had C and CE and C1E entitlement on my licence since they issued that plastic one. I wrongly assumed for years that this was provisional entitlement.
I decided I wanted a +7.5t motorhome, I know that the driver licence issue for motorhomes is a grey but decided that I would take the C test anyway. I booked a theory test and crammed for three weeks, and learned all about knots and tarpaulin lashings etc
When I turned up to take the theory test the examiner checked my licence and looked at me as if I was stupid. I was.

He explained that my licence entitlement me to drive
anything. Even those 40 ton monsters as long as its a draw bar trailer and not articulated. I discovered that I had these licences via grandfather rights after passing the old HGV3 test in the Army back in 1978.
So the heaviest vehicle I had driven in the last 20 years was my 4.5 ton AutoTrail I then jumped straight in an 11t RV. To be honest after 30 minutes of getting used to the dimensions it was a doddle.
Do I think that you should have to take a test before you get in a motorhome over 3.5t? YES most definitely. Am I glad I didn't have to? you betcha