start with the worst first, use a piece of angle or channel to spread the pressure from the jack on the backside of the rail to minimize the collateral damage. reinforce the rail at the highspots on the outside with some large C clamps and then chain the clamps to the bus frame to hold them in place. start jacking the worst damage out and work the outside of the rail with a block of wood and large hammer (sledge) as the rail starts to straighten out you will want to start working the skin where it is kinked up near the floor, lots of little hits to relieve the internal stress in the metal, you don't want to leave a bunch of hammer marks to fix, the metal moves when you hit it even when you can't see the progress with each blow. keep moving and working the worst spot.
you probably should invest in a couple of body hammers and dolies, basic harbor freight set $30 IIRC. body hammers have a flat face and don't dimple and strech the sheetmetal like other hammers will.
you can feel straight better than you can see straight when your up close, lay your hand palm down fingers extended on the sheetmetal and run your hand fingers first over the skin and you can feel the highs and lows.
steel has a memory and wants to stay where it is you will need to push the rail past straight to end up straight, you will start to get the feel of what the metal wants as you work with it.