Keep putting it in the trunk before you get there. The front gate MP post has no way to securely store your firearm. Post will direct you to the MP office or Conservation/Hunting lodge where it will be a bureaucratic mess logging in the gun, and later checking it back out again.
Just say no, put it in a secure lockbox in the vehicle, and don't worry about it. If they don't know, they don't have to jump thru hoops to do something they have no facilities or procedures to handle. Nada.
Vet, 22 years, IN/MP, I've been on plenty of posts and worked Ft. Benning six months there. In all the train up and duty, no mention of civilians needing to check in firearms or any place to do it. WE did for duty guns, a 45 minute ordeal before and after shift. Made for a long day.
It's really no different that any other small town, we don't check our firearms in at the police station just because we happen to be visiting.