I have towed our 2.7t van on some of the Google suggested route, south east from Willunga towards Mount Compass.
Refer link -www.google.com/maps/dir/Goolwa,+South+Australia+5214/McLaren+Vale,+South+Australia+5171/@-35.4090426,138.5511115,11.54z/data=!4m14!4m13!1m5!1m1!1s0x6ab6b55552406141:0x5033654628eb200!2m2!1d138.7837286!2d-35.5028883!1m5!1m1!1s0x6ab125e53337b26d:0x5033654628eb7b0!2m2!1d138.5448927!2d-35.2192433!3e0%3D%3D
I consider that it was a good road, though some parts are a bit steep, but not dramatic. I used my auto transmission in manual mode on the steeper pieces.

The Google suggested route from Goolwa goes through Currency Creek, Mosquito Hill, Mount Compass, Willunga Hill, Willunga and then to McLaren Vale. Google Maps StreetView shows good road surfaces along the route.

Enjoy your trip as there is plenty of wineries and dining to sample. We liked Maxwells for some Honey Mead and of course many other winery and distillery choices as well. The boutique food stores are great too. I think the Foodland store has a good selection of local cheeses and produce.