You don't just fill up for free and go...
More than likely if the base has a CNG station it is for the govt vehicles that are running on CNG and not for the typical consumer (military or not). The fill station is probably at the GOV/Vehicle Dispatch and used only for those vehicles. The pumps usually require a code or special key (typically attached to each vehicle key chain) to even turn the pump on.
In any event, the government is def. not giving away free CNG, no matter how cheap it is.
Don't know if you are military or not; I am retired Navy. The two Naval stations near my house will let anyone with a CNG system fill up for free; please do not presume anything said is false without first checking your facts and sources.
I asked a couple of years ago; the main reason for this is most of the CNG fleet is aging and being sold, without viable replacements (may or may not be completely accurate). To fill, there is no key, and I don't believe you have to let the attendents know you're getting it (all this may have changed now). I actually had to ask twice just to make sure it was that simple.
I would love to find some place that the government is giving away free CNG hahah . No they will not give it away unless you happen to be a none tax paying immigrant etc then they will give you a new business and car house what ever you heart desire. Then we the tax payer will buy it for you.]
As for being a "none tax paying immigrant etc" , "Then we the tax payer will buy it for you" don't forget this is only on military installations (at least near me); we all pay for it, no matter if in uniform or out. For the sacrifices active military make I think you can safely call this a small "thank you" and be on your way.
Anyone care to take me on about this (and, by proxy, millions of active duty and veterans)? Yeah, it burns me up when people automatically heap us with "non tax paying imigrants". Everyone has opinions, and you have the right to express them; just remember who defended that right for you before you start callously throwing rhetoric at us.