"I prefer not to run a thick extension cord" ...
But, is necessary!
if you try to splice in additional length on the 5 volt DC side, the voltage drop will be enormous and the camera will not work, unless you use a very fat wire - bigger than the extension cord!
Google Ohm's law.
I did add about 2 feet to my dc cord, and already the voltage was getting marginal.
My qualifications for explaining this include BSEE and MSEE.