Look at the suspension, make sure the welds are good.
Check the tires, especially the date codes. If old, try to work a deal on reduce price to cover the cost of new tires.
Look at the corners at the slide-outs, make sure no cracks or damage.
Check the ramp and make sure it is strong and not soft.
Check the bed lift, make sure it works.
Also, don't forget the roof, especially all the sealant around the edges and vents.