Before, when I didn't drive the truck for a day or two, it would be hard to start. I would usually have to let it turn over for a few seconds, turn it off, and then switch it on again and let the motor turn over for a few more seconds before it would finally start. After I would drive it for a while and cut it off, it would start right back up. Now it's just turning over and not starting at all.

 

do you have a lift pump pressure gauge installed? if so, whats the pressure?
if no gauge, remove the lid to the filter canister, see if it is remaining full. you may be experiencing a siphoning action that is draining back to tank when off. check that and come back here with results...

 

I don't have a lift pump guage but I took the lid off the filter canister and it was full. What do I check next?

 

can you hear the lift pump run when the key is on?

do you get the wait to start light on the dash?

 

I could hear the lift pump running when I initially turn the key on and the wait to start light is never coming on. I just turned the key again though and after turning over for a few seconds it started up. I guess I'll have to figure it out next time it does not crank.

 

sounds like your preheater circuit, or some component in the circuit is dead. open the fuse box under hood and check to make sure the fuse for heater is good(under the lid is the label)
if you're electrically handy and have someone to help. get a mutilmeter, and check to make sure you're getting 12vdc at the heater grids. on the intake horn, there are 2 studs with large gauge wires going to them. those are the positive wires.
if you get voltage, then check continuity of the grids themselves