C15 fuel system leaking down?

During this slow spell we are experiencing, I have noticed that if my 2004 MBN C15 stays parked for more than two days, the fuel system is apparently leaking down. It will crank, idle at approx. 500-600 rpm with an "air bound" type of miss, then speed up to normal idle of 750rpm. Two days of inactivity, the miss is almost not noticable. Four days of setting will cause this abnormality for approx five seconds.

This is something new, and maybe just a coincidence, but seems to have started this after I recently had my truck serviced by a local (and usually VERY reliable) company. As usual, Cat filters, and I double checked both fuel filters for tightness, (hand tight, didn't put a wrench on them).
Will probably R & R the filters tomorrow, and if that doesn't solve the problem, wondering if I should replace the check valve next. If so, where is it? Any trick to the R & R?

Thanks in advance.