I bought my 77' 31 foot Sovereign
for $800, but it had a newer Carrier AirV AC, a newer Iota DLS-55 Inverter, a newer water heater and water pump. It looked just like yours on the inside. Mostly gutted, but that is the way I wanted it as I am in the process (slowly) of turning it in to MY Airstream. All systems turned out to be in tact. I didn't necessarily know it at the time other than the 120V system, as the previous owner was using it to sleep in at his company and running the AC at night. It's a long and tediou$ proce$$ to get from that condition to roadworthy, but if that is what you want to do... $500 is a small investment. If you get overwhelmed, which happens easily, you can probably get your money back fairly quickly. Or, just use it as a storage shed! You can't buy a shed that cool, that size for $500!
As a person with an aluminum addiction, I say grab it.