Our 1984 31' 310 Classic motorhome has a GVWR (gross vehicle weight rating) of about 14,000 lbs. Our 1974 20' Argosy motorhome has a GVWR of 10,500 lbs. I'm pretty sure the 20' Argosy has a typical curb weight of about 8,000 lbs so it would appear that even the smallest Airstream motorhome is going to be over your 3-1/2 tonne limit.
Now if you're referencing the metric tonne (2204 lbs) (3.5 x 2204 = 7714) then it appears that the 20' Argosy would come close but still would be a little over your limit.
I've never had the opportunity to weigh our 20' Argosy so can't say if it would fall within the 7714 lb limit. Also ours has a 454 gas engine and I believe a diesel engine would weigh several hundred lbs more.
To the best of my knowledge all of the older Airstream motorhomes were based on the Chevy P30 chassis. I believe the very last Classic style motorhomes that were made were diesel pushers based on a Ford or some other brand of chassis however these were in the 35' and longer category.
Hopefully someone will chime in with some better information for you.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)