92 Fleetwood Bounder only starts with aux boost

Just remember that having voltage does not equate to having full power. My power solenoid( activates when ignition is turned on, had pitted and worn contacts inside the case. They would read 12.xx but would not carry the amperage needed to start the engine. As stated, clean terminals first, then test and eliminate bad parts.

Get the battery load tested, most chain parts stores will do it for free. That can show a lot.