Ok Peter
I'll try to work through this with you
1) no dash gauges and or no start condition
This is not an uncommon fault it is usually caused by the ignition solenoid located in the electrical bay being intermittent or faulty.
This solenoid is an 85 amp continuous duty solenoid which controls the ignition power to all the ignition operated functions it should pass 12 volts to the left hand buss bar with the ignition turned on ( that buss bar is all ignition feed circuit breakers.
2)
The leveling system may be as you say a bad switch or possibly a wiring connection .
( Which system do you have RVA or SMC )
3)
rear hour meter runs all the time the battery is connected .
The hour meter should only run with the ignition turned on this problem and your rear run cabinet gauge lights may be the same problem as those lights should be the same way this is likely the wiring right at the rear run / service panel check it against the wiring diagrams for a wire which is in the wrong location .
4)
Oil pressure gauges don't read the same.
On these coaches the oil pressure gauges don't share information with the engine ECM ( the ECM has its own oil pressure sensor)
On your coach on the right hand frame rail beside the engine you will find an oil line with a tee and two oil pressure sending units screwed into it
These are the oil pressure sending units one for the dash gauge the other for the service bay gauge ( I'd replace both you can get them from almost any parts house )
5)
The rear oil pressure gauge is always lite up indicates the circuit is being bad fed possibly again check the wiring on the back of the service bay panel for inappropriate wiring.
6)
Fuel gauge pegged
This is most likely a bad connection at the sending units as mentioned in my last post ( clean those connections and try it again )
7)
The Tach at 1000 rpm when not running
For these gauges that is normal or acceptable however if they fluctuate or bounce while the engine is running the problem is most likely poor ground to the gauge.
Coolant temp gauge doesn't move off cold
This could be one of several things number one the engine has to warm up this can take as long as 30 minutes depending on ambient temperature
You could also have a thermostat or sending units failure.
9)
8 volts at the isolator chassis battery side I assume
This is likely the solar panel input you can verify this by covering the solar panel/s with a blanket and rechecking for voltage
10)
The boost solenoid comes on with ignition and gets warm
The boost solenoid in your coach will be a White Rodgers continuous duty solenoid mounted at the isolator panel the control switch for it should be the center one in the rear row of switches on the driver left console it will be the momentary switch
Your coach may be wired differently to allow boost operation on ignition operation to aid in start up ( your going to have to look into that one)
11)
Engine preheat light comes on after startup
This is normal as the ECM will cycle the intake heater for a period of time during cold weather start up to aid in engine warm up
( The period of time it does this varys with the engine temp ( colder =longer)
12)
Solar red light
I'm not familiar with your particular solar charge controller however these coaches usually had two solar panels one for the chassis battery which had no charge controller and was so small it was almost inconsequential and a larger one which charged the coach battery bank it two was not very large and was intended only to top up a static battery bank when not in use.
The red light may be a fault light or a charge indicator either way it would be best no have a coach battery in the system before looking into that.
13)
Turn signals ake no noise
This is normal as the flasher unit is mounted in the electrical service bay you won't hear it anyway.
You could install a set of beepers on the dash indicators if you want an audio output
This should give you a good starting point .
My advice would be to at least install a battery on the house battery bank before going any further as the inverter will. Set faults if powered up without a battery bank to monitor or charge
Hope some of this helps
Eric