2007 C1500 Chevrolet Pickup - my keyless entry only works at the door. Is there anything I could check?

 

could be a couple of things that I can think of, one would be the remote battery getting weak and not able to send out a strong signal, two and the less likely of things is it could be something going on in the bcm or the receiver antenna in the vehicle that the remote sends the signal to.
cheapest thing to start off with would be to replace the battery in the key fob. next, if that does not fix your issue I would see if you have any codes relating to the bcm.