Dave.....Just buy some fuel line that will fit the line on the generator. I would use a five gallon fuel jug since you say it will often run for two hours. This way to could run it for up to 8-10 hours. This may not be the fix, but if you're handy, it's a lot cheaper to eliminate some issues at home before spending $100.00 an hour. Make sure you put a load on it, like running the A/C.
Here's another thought and it may be hard to remember the circumstances. Over the years I remember some Monaco pick up tubes getting pinholes in them. Even though your tank may have enough diesel, the pinholes could be at the 3/4 or 1/2 mark. Have you ever run the generator with the main tank full to the top and had the issue.