An AC50 pilot failed to verify that the fuel cap was installed after a fuel stop. Because the cap was not installed fuel rapidly siphoned from the interconnected tanks resulting in critically low fuel at the destination.

Narrative

I was flying this route for the last 3 nights and it's pretty simple; there and back deal. I always get a top-off at my destination since it's about $0.50 cheaper. I noticed two line guys working on my plane tonight; but figured they did the job. Did a quick glance to check the cap and it looked like it was there; but it was dark so I wasn't 100% sure. I did not double-check the cap. My cargo showed up so I fired up the engines and took off.About 25-30 minutes into the flight I saw my gauge show 90 gallons. The Commander has 5 fuel cells all interconnected; one fuel inlet; 2 outlets to the engines in the central tank; so the cap is pretty important since one tank can't be isolated from the rest. Looking back at my gauge; I began to think it was showing right. But another 10 minutes in; I was down another 20 gallons. I was using around 120 gallons per hour; the normal flow is 30 per hour. I was immediately thinking for outs in case I wouldn't be able to make it back to base. In the end; I landed with about 20 gallons in the tank or about 10 minutes left.My inattentiveness to doing a through pre-flight during my turnaround and the fact I did not physically check the fuel cap caused this event. Also when I realized I did have a problem; I continued the flight rather than divert pushing the safety margin.