First rule of troubleshooting, check the easiest things first.
Easiest thing to check is, now that you've filled the propane tank, is the generator getting propane? The person who fills your tank is required by state propane gas codes to shut off the solenoid switch first before filling the tank. He's not required to turn it back on. So check the switch.
If the switch is on, then since you ran the tank completely empty there may be air in the gas lines. Easiest way to check this is light a stove burner, partly because it's the only appliance where you can easily see the flame. If there's air in the line, it will take a while to light the burner because first the air has to be bled out of the line. Check this with the sliding side door wide open and the roof vent turned on. That way you ensure there is no propane buildup inside the van while you're mucking around with the stove burner trying to get it to light.
If the stove burner lights and stays lit, then try cranking the generator again, but keep cranking until it catches, but no more than 30 seconds at a time. If it won't crank after three tries, check the generator oil. There is a low-oil cutoff switch inside the engine, so if it's low on oil it won't start.
On Edit— As soon as I posted this, I thought of something else. Make sure your breakers are on, but all appliances that use 120vAC are turned off. The generator will not start under a load. Wait for at least a minute after the generator starts before applying loads.
If you go through all of those and you still don't find the problem, then it's time for someone else to step up and take it from there.
Engineering: Finding complex solutions to simple problems you didn't even know you had.