[Richard]

Ok, first you have a big leak. A leak so big that the 120V or 12V output will not overcome it. Also, with a leak that big, both of the little compressors may have worn themselves out trying to keep up.

Second, you are exactly right about turning on the key. When you turn on the key it opens the travel solenoids to the airbags. They usually will take about 90 psi to "float" the coach. If the system pressure is less than the pressure in the airbags, then air will flow from the airbags through the travel solenoids, through the height control valves and into the system.

Start by piping the 120V compressor into a deadhead with a gauge. Determine if the pump is actually capable of building pressure.
On the 12V compressor, confirm that you have 12V power on the pressure control switch at the pump. The switch is commonly referred to as the well pump switch. It is usuall a grey box about 2 by 3 inches. Wires will originate in that switch and go directly to the pump to turn it on and off. Start the diagnosis by determing if the swtich has power.