[@Klomster] Well, as I said; a Cadian armored company, which the group of guard on this ship would have to be, as Vespin is a Cadian officer, would have at least a half-dozen leman russes. They wouldn't use an entire ship to transport anything less than a company, as anything less wouldn't be worth making the trip. If the ship was alongside a larger retinue of ships, as part of a larger group, then I'd be inclined to agree with you. However, it's not.

As for why the tanks would be in there; The guard company itself would need somewhere to keep Its weaponry. No point in having guardsmen packing lascannons and heavy bolters to their living quarters, after all. So, the logical place to store the weaponry of the entire Cadian armored company would be in the ship's armory, as that's exactly the kind of things its made to hold.

And the tanks would be in there for the reason I described in my original post of the room; so that the heavy weapons platforms being stored in the room could be lowered down into the vehicles waiting below.

However, I could very well be wrong. JB has provided next to no detail about the group of guardsmen (note, guardsmen, not Imperial Navy troops) that are being transported on this solo ship. We don't know why they're being transported, or where they're going. However, I'd assume any group of guard requiring an entire ship as transport, as well as having an attached penal legion, would be carrying enough weaponry and artillery to outfit the entire company, at least to the point of making worth the effort of moving around the man power. After all, not a lot of sense in hauling a bunch of soldiers to a battle field, if you don't bring their guns, armor, vehicles, etc. with them.