Thanks for all the replies and suggestions. The air brake is on and the trans is in neutral as they should be.
I have the key in the accessory position, which is the correct procedure according to the manual. There was no mention of a lockout mechanism except that I know the slides will not operate with the ignition in the on position.
The suggestion about a power controller for all three slides makes sense to me. I will start looking around for that. Not sure about solenoids. The ones that I have found near the inverter appear to be for connecting coach and chassis batteries, but there are more than a couple of them, so I'll look for one labeled slides as this motorhome seems to have labels on 90% of the wires and fuse panels.