The PO of my 85 Westy Weekender used to tow a 16 ft. Hobie Cat with a Sun Fish, that's a small single hulled sail boat, rigged under the Hobie along with his family and all their gear into the mountains near Yosemite. I know third gear was used a lot but they made the trek annually with few if any problems. I'll bet he was hauling more weight than your talking about.
Best of luck,
Paul Rogers
85 Westy Weekender