Hi, I have a 1988 Rexhall Airex with the 460, so mechanically it's pretty much the same rig as yours.
For a manual for the engine and chassis, you need the manual for the Ford E350 van. Haynes and Chilton both do one, but my recommendation is to buy the online one from Mitchells here....
Do it Yourself Automobile Repair Manuals - Mitchell 1 DIY You want to specifically get the "E350 RV Cutaway" version.
You can open it on a tablet or phone when working on the rig, or print pages, wiring diagrams etc as needed. It's a lot more detailed than the home garage type manuals from Haynes or Chiltons.
For your issue, I'd check in the following sequence.
1) Check all connections are tight and not corroded. A cheap probe with a 12v bulb in it to check continuity is your best friend here.
2) Disconnect the wire from the small terminal on top of the solenoid. Clip a probe light to ground and put the tip of the probe into the end of the wire. Have a helper turn the ignition key to 'Start" The probe should light. If it doesn't, your problem is upstream of the starter solenoid. That's a wire run from the battery to the ignition switch, from the 'start' position of the ignition switch to the fuse panel, from the fuse panel to the transmission position sensor, and from there to the relay.
3) Connect a jumper lead (Doesn't have to be very heavy wire) from the positive terminal of the battery to the small terminal on top of the solenoid. The engine should turn over. If it doesn't, listen for a 'click' from the solenoid as you connect and disconnect the jumper. If you hear nothing, double check that the solenoid is grounded - that is accomplished through the screws that fix the solenoid to the firewall, so just remove them, wire-brush them and the holes they go into, and screw them back. If you still hear nothing, the solenoid is probably bad. If you hear a click, connect a 12v test light, or a voltmeter, to the lug of the solenoid that does NOT have a cable going directly to the battery. When you apply 12v to the top terminal you should get 12v on your tester too. If you don't, the solenoid is bad.
4) While checking stuff, check that the starter itself is good. Clip one end of a good heavy duty jumper cable - the type you jump start your car with - to the positive terminal of the battery. A heavy wire goes from that positive terminal to one of the two heavy lugs on the solenoid. Press the other end of the jumper cable to the OTHER heavy lug on the solenoid. The engine should turn over. If it does not then the starter, or wiring to it, is bad.
5) To check if it is the starter or the wiring, you'll have to get underneath. See where the heavy cable from the solenoid connects to the starter, and VERY CAREFULLY press your jumper cable to that connection. If you touch the jumper to the body of the starter, or the engine or frame, you're shorting the battery direct to ground. Noisy and sparky at least, and if you are unlucky it CAN weld the jumper cable clip to what it touches, so like I said.... VERY CAREFULLY.
Here is the wiring diagram for the starting system. As you see, it's pretty straightforward.
https://www.dropbox.com/s/1hr881naty...rter.jpg?raw=1
Richard