Black oil goop in secondary air tank

I just purchased this truck about a month ago. It is a 2013 Durastar with the 6.4L Maxxforce 7 engine and air brakes. I have been going through the truck since I picked it up in California. It has 93K miles and a good history of dealer maintenance.
The batteries gave up the ghost last week and I removed them and they were the original International batteries with date stickers of 10/12. While I had the step removed I decided to swap out the butterfly tank drains with pull type drains. The secondary tank (wet side) started oozing out a ton of black , thick oily goop. I let it run out for an hour and ended up with about a pint of crap.
I am assuming the original owner (his drivers) did not drain the tanks at the end of the day and this is an accumulation of oil build up of years.
Is it possible to drop the tanks and clean them? Or, should I just buy a new secondary tank and move on?
I am guessing the air pump (Bendix Tu-Flo 550) is bypassing oil and needs to be the next item to troubleshoot. Also, I was surprised to find the primary air line from the compressor to the tank did not go to a dryer first. Every truck I have ever owned or worked on that has air brakes always had a dryer in the system.