There are leg exercises people with bad knees can do. Squats are possible you just can't do them deep or with high weights. Also anytime you do something with bending, the knee should never be out further than the toes. That puts alot of pressure on the knee.
Step ups, partial squats, adductor/abductor machines, calf raises...all can be done with bad knees. You just have to be careful and not over-exert yourself.
Losing weight will also help relieve alot of pressure on your knees as well. Being over 300lbs has to be one of the biggest culprits. If you could stay smart about both diet and exercise for a long period of time and get yourself down to the 250 range, which wouldn't make you look weird, I'm sure alot of your ailments would significantly dwindle.