Don't worry, you're not alone in having a soggy back end. The same applies to my car too(!) However, when I bought both my previous two towcars, I was told by biased and unbiased sources that the back end should not sag, but when you have 4 people in the car, 80kg on the towball, a boot full of awning and barbaque, and 4 bikes on the roof, the steering has felt light and the tyres have been in the arches.
I partially solved this on the first car - a Vauxhall Vectra hatch - by fitting the rubber doughnuts to the rear suspension, which improved stability and made the car ride higher when towing, but affected solo ride and body control.
On my current car - a Rover 75 Estate - I have fitted progressive auxillary springs from a company called MAD, and they have affected the solo ride little - if anything, it is more composed - and the car now rides higher when loaded, and feels less wallowy when towing. I am about to tow to France tomorrow, so when I get back I will let you know how they work.
If it is an older V70 you are looking for, then fitting newer springs might help. However, I would recommend the latter, for