On Campendium, you can filter campground results to find military campgrounds. On Roadtrippers you can see if a place is a military campground in the "See more details" section, but it is not possible to filter search results based on that data. This makes it difficult to find these campgrounds while planning a trip. See Fort Belvoir Military Travel Camp, for example: https://maps.roadtrippers.com/us/fort-belvoir-va/camping-rv/fort-belvoir-military-travel-camp