I bought my 2021 Superb SEL 2.0 TDI DSG 4 x 4 last October with only just over 20,000 miles on the clock. My caravan is a Bailey Unicorn Cadiz, which is around 1500 MTPLM. I've only towed it from the Dealer to my storage compound, about 15 miles on A and B roads. All I can say is that it towed very well with no problems, much better than previous vehicle, a 150 bhp Mazda CX5 diesel. I'm no mechanic, but with your mileage, worn clutch has to be the prime suspect. Good luck with sorting it, particularly if you'll be pulling it up the steep ramp on the ferry to France. I'm going to the North Yorkshire Dales in May (wife won't go to France anymore) and I have no worries about the Yorkshire gradients.