There is a possibility your coach has a fuse panel on the drivers side compartment under the drivers side window.
Open the compartment and see if there is a screwed on panel covering the fuse panel.
There is a main ignition fuse often 50 amp size, if the power step does not work. this is one of many of the items powered from this fuse. Also looks like there is a ignition relay on the panel. Might control the starter circuit.
I also have been thinking that the ignition switch might be the problem. you mentioned that when the ignition switch is in the run position all the circuits powered from the switch work in this position, if I'm reading things properly.
One way to test the starter circuit is to find the exciter supply voltage at the alternator and see if there is voltage there or not. If you have no power on that terminal, then look and see if that wire got grounded and that is also grounding the starter relay power. Should that happen then you could not start the engine.
One little wire in the wrong place and things do not work. You did get the starter repaired and replaced just before this issue came up !!!!!
Should you fine that the alternator energizer wire got grounded in error, then you might want to check the diode in that circuit ti see if it got damaged. Stranger things have happened.
NOTE ! The alternator exciter voltage and starter voltage are only present when the key is in the start position.
You could check out my Alternator issues photo gallery at this link.
Still looking for additional information.
The VIN number for your coach.
VIN number location.Try turning the front wheels hard left
and look on the outside of the right (passenger) side frame rail behind the
right front wheel.
With it we might be able to locate some additional information.
This link might be worth looking into.