1999 Ford E-Series Van Starting problems

I have owned this van new, and have taken good care of it, with regular maintenance and oil changes.

I just replaced the radiator two weeks ago, and everything seemed to be working and running fine. I went on vacation for a week, and the vehicle sat. I went to start it, and the starter engaged, but didn't crank. I put volt meter across the battery (which is only 2 years old). Connections are clean and tight. Voltage was about 12.7 nominal, and with the lights on and attempting to crank, fell to a bit over 7 volts. I didn't think it would be the battery, since everything else seems to function fine, and it is a 60 month battery. Thinking the starter was 10 years old, I replaced it out of hand, thinking it was seized or defective, but the replacement yielded the same results. I had the core checked for function, and it worked perfectly. I am fearing a much more serious problem.