You did start with your troubleshooting you replaced the battery.
There are so many things it could be, first I would make sure it is in park.. could be park / neutral switch try it in neutral also just to rule out that it isn't activating park switch.
Could be the ignition switch
By pass the ignition switch by jumping starter
when you say the main disconnect above the door I am guessing that would be to disconnect house batteries.
Someone will come along with more info.
Jerry & CP (copilot) Joyce RVM 109
2000 Coachmen Mirada 340 MBS
Chasing the Rainbow