They call them a 12-volt battery, but a fully charged one should read at least 13.5-VDC, even under load.
One that only reads 12.4 volts across the battery leads is not charged and probably will not crank the engine. But, you should at least hear a click.
As to your engine starter solenoid wiring, I assume your engine and drivetrain is Chevrolet, so check with your local Chevy truck dealer and see if they will share some info. Almost all of the motorhomes have drivetrains, transmissions, dash components and wiring that is standard with the truck manufacturer's and they can service yours if you need this done.