The weight on the front wheels wouldn't be any different than when going down a moderate downhill; the only additional weight would be due to the shifting of the center of gravity closer to the front. If you're carrying a piece of furniture with another person, your end doesn't get vastly heavier if they raise their end a couple inches. It's the same physics for a vehicle as for a sofa.
It makes no sense to tow by lifting the back wheels, though; it requires the tow truck to carry much more weight, and also requires it to have a wheel lift that extends back much further than would be needed to lift the front wheels. (I think it would be in excess of ten feet for my class C!) If they were to try to lift it by hoisting the (extended) frame rather than the axle, they could very well risk frame damage as it's not necessarily designed to support the full vehicle weight when lifted from that point. I don't disagree in the least with what wildtoad says: front wheels or flatbed only, and disconnect the driveshaft if pulling from the front.
