Having said that, reading comments like this, I sometimes think it would actually be great cover. Because you have respected people, like yourself, unequivocally stating that it couldn't possibly be an NSA van.
But, to say it again, I agree that I don't think the NSA would need to do this. My above line of reasoning certainly doesn't hold too much water under serious scrutiny.
You can second-guess that, but I think past this point, we're reenacting the duel between Vizzini and Westley.
It's like the spies working in embassies that were easily detectable despite an elaborate cover because they used the car that the previous spy left behind when they went home.
So I guess the reveal is that it _is_ a real sewer inspection van, but the NSA has legitimately been inspecting sewers for years to innoculate themselves from suspicion?
I guess they must be down there looking for rodents of unusual size.
You can either disguise your operation as a goofy sewer inspection van and hope you trick every single person who notices it into second-guessing themselves along the lines of "surely the FBI would be more low-key than that..."
Or you can just be low-key in the first place, end of story. I assume the tech in the modern day (as compared to, like, the 80s when this trope was born) is advanced enough to facilitate this option.