What is date on all the batteries..? Both coach and chassis batteries ? Can you start gen? If so.. start gen.. leave running for 20mins and then press and hold aux start button for 10mins and while holding down try to start eng.. then drive asap to closest place to get battery and have them test.. if your chassis battery is bad.. it can burn alternator up too.. and often does..
If chassis battery is new.. test voltage.. what did you get? Could be bad chassis battery disconnect switches.. that is one of the large black boxes under hood.. above battery.. they are often can be taken apart and cleaned.. they get very corroded over time and need cleaning. But use all safety precautions.. that means.. negative cables removed from all batteries, coach and chassis.. no shore or gen or solar.. it's very tight in there and you might short out wires..
Use cellphone with camera and flash and record all wires 1st before you touch them.. and mark where they go.. they all need recleaned and resealed.. take your time.. it could be time for new starter.. and there are tests that can be done for that.. could be bad ignition switch.. they ove heat and burn up..
Good luck and keep us posted
2000 southwind storm, workhorse custom chassis with 7.4l vortec