I'll just run some floodlights in those tunnels - I'd imagine that sun-phobic zombies wouldn't be able to tell the difference between sunlight and artificial light (since they are dead and all) - plus those access doors are windowless and easily sealable.
Probably that building owned by ATT on Mifflin at Henry. That thing seems locked up pretty tight. And there are plenty of places nearby you could gather supplies during the day. Otherwise the Court House or Police Station for the ammo stockpile.