You can also join if you have a sibling, parent, or child who is a member. They also serve Local Govt Federal Credit Union, where you have to be a loval government employee to join. However, LGFCU also accepts volunteers for city/county government (such as libraries, schools, volunteer FD, etc). They should have a form at the credit union that you can get signed by whomever you're volunteering with (or just have them write a letter). I know it's a bunch of hoops, but once you're a member, you're a member for life. They will do 1 personal residence, 1 vacation home, and 2 investment properties per person. So double that if you have a spouse and finance them in one name only ;-)
I know one guy who has 8 or 9 properties financed there because they will always do a new loan for a personal residence. So he just keeps moving every couple of years! The credit union doesn't sell their loans either.
You should be able to join Pentagon Federal since you're a federal employee. They did a HELOC on one of my rentals last year.