1999 Chevy Suburban Starting Issues

This truck is typically one of the best I own, dependable for years and right now I'd take it on a road trip to anywhere. Lately though after more than two days of not starting it, it tends to not start. Battery is new, all other electrical parts have been tested and passed, but it still won't fire up. (Sometimes)
It turns over fine for awhile until it starts or I have to boost it, either way it starts then runs fine Forever, until I let it set more than a couple of days?
What can this be? Fuel filter maybe?