Starting Issues…Fuel system?

[MotherTrucker]

I have a 2008 de-mandated ISX. Just hit a million miles, previous owner did an out of frame on it at about 750k. Bought it in Jan, and have been “getting it right” since. It’s almost there! I took it to Mr Hag about 6 weeks ago and had him go through the tune (and rebuild the jakes and do an overhead). It’s always been a little sluggish to start, although not to the point that anyone but me really notices. Just takes a second or two longer than I feel like it should. On cold start the rpm’s will vary 25-50 a few times until it starts to warm up (noticeable on gauge at low idle, less at high idle). Every now and then when I turn the key it will act like it’s starting for a second, then quit…have to turn the key off and try again (this has happened maybe 4 times since Jan). Within the last week, it has given me the orange check engine light 3 times on startup for 3-5 seconds after firing up, then it goes away. I haven’t had the code read, but will try to get that done this week. I had the check valve replaced about 2 months ago. Last thing to note…my fuel gauge is wonky. Will go from full to 3/4 after 50 miles, to 5/8 after 100-150 miles, 1/2 at about 400 miles (tanks are 75 gal tanks, and I generally get around 6 mpg). Below 1/2 it’s fairly accurate. If I stop driving and let it idle, the gauge will go up to a more accurate reading of fuel left vs miles driven. The truck drives well, has plenty of power, haven’t noticed any lack of power or surging while driving, but my fuel mileage does seem to be more variable than I would expect. For example I got 7.3mpg yesterday through Washington with a 14k load on, through MT and ND today was only 5.7, but it was windy. So…my question is, are all these things related? Obviously the starting issue is my biggest concern, but any advice is appreciated