Agree if no leaks, soft skin is tight, raises and lowers properly and when latched down it is square and secure. And the price is good i wouldn't let it stop me. THe alum frame should have a slight bow/arch from front to back. Drop a long level on each beam. Put some palm pressure on and along the beam, listen and feel for crackling or obvious breaks. That feel and sound will be different then the sound of the alum skin flex.
What year and price?
Is everything else good?
If the skin is just bowed down around the beams and is consistent it probably had a snow load on it for a extended period. The alum skin is soft. The weight on just the skin could have slightly elongated the screw holes. If all else was good i would loosen all the screws in the skin flats. Just enough to chalk under. Go inside and gently push up to reverse the downward skin bow. Put a small amount of self level roof chalk under screw head. Allow to cure almost all the way, retighten screws. Top each screw head w same chalk. This small project should only take a few hours. Again if year price and no other damage needed immediate attention. Just my humble opinion.