We have had a few do this same thing. I would replace the check valve for the return. Its on the driver side injector area between 5 and 6 injector. It has a banjo type fitting on top.
We're having the same issue with a Mercedes powered C2. Bus ran fine, parked over a weekend, will not fire over. Full tank of fuel. Pressurized fuel system starts up. Sits overnight, will not start.