I had similar damage to yours around the wheel wells. Check out my build thread from the link at the bottom of this post, but here's a synopsis of the repair I did: https://imgur.com/a/Vp4Xpv6
. On your bus, the section between the two wheel wells looks fine so probably your repair will not need to be as drastic as mine was. Just get in there with an angle grinder and cut through the rusty stuff until you get to relatively healthy steel, square everything up and weld in sheet metal and angle steel to repair/replicate the original structure. Don't worry about the bus collapsing around you as you cut - as you can see, a bus holds itself up around that area.
My step well is nearly new, having been replaced by the previous owner due to rust, so I can't help you there. But it does look fairly easy to cut out the rusted parts of the step and weld patches over it. Buses often use a piece of rub rail on the underside of a step to stiffen it, but a piece of angle steel would serve the same purpose. Or, you could buy a whole replacement step unit for, uhhhhhhh, a mere $770
. Never mind.