There are US military optics from the 1960's that still work. Night vision is incredibly durable, after all, they hand them to 18 year olds and say go get 'em tiger. They get dropped, left on during the day, get blown up, etc.
The D-300 is a commercial housing with anything from a US gen 3 tube to a commercial gen 2 unit. The tube is everything, the housing just holds the tube, battery and lens in place. US tubes are very durable, but some do go bad. The commercial gen 2 tubes can be comparable to the US made tubes or Eastern European tubes that do go bad from poor material or workmanship. Shine a flashlight in the rear lens, you can sometimes read the info on US made tubes. Although many tubes just have a simple stamped serial number on the top or side, which you won't be able to see.
Luckily for you, housing can and do go bad. I had one cheap housing that the battery problems caused by the housing, but the tube was fine. I'm positive the D300 uses the same size tube as a PVS-14, so you can switch housing.
One way to test the tube is to remove it from the housing and run 3 volts directly to it (CR123 battery). There are assembly/disassembly videos on Youtube, particularly of the PVS-14 and the D300 works the same way.
Now the bad new, tubes do go bad. Everything from the power supply to the actual tube has a leak internally ('gone to air'). If the tube is bad, you can sell the housing for $250-500 or so. I'm not sure on the current going rate, it's been a few years since I've assembled a unit.
