We've had MPI (fuel injected) boats that love to have this problem. However, their pumps are not in the tank and their fuel systems do not recirculate.
I am, however, having the issue with a generator.
If I had a modern 8.1L, I'd see if it's setting any codes (check engine). If not setting any codes, I'd probably get a OBD-II scanner that can show me real time data. (Note, if you use an Android phone, you can do this for under $30 total... )
If vapor locked, you'd measure near zero fuel pressure.
Last way to check - and this is more dangerous: Find the schrader valve on your fuel rail. Let it stop, depress the valve. If vapor locked you'll get air. If not, you'll get fuel. Note spewing high pressure fuel on hot motors is dangerous, so the standard disclaimer of - have a mechanic do it is in order...
To gather more evidence without all of the difficulty above:
1) Switch to running your coach AC off the genset.
2) Drive in the morning or in the evening after the sun is down. Pavement temperatures right now are crazy, easily over 120 degrees.