Your options for the SP are limited. I might suggest you get a pistol safe that can be secured (by bolts, cable lock, etc) to something like the heater, bed frame, or some other object that is essentially not able to be moved or taken apart without substantial tools. That and put at least some effort into concealing its presence from maids and the like. Or see if the hotel can give you a room that provides an in-room safe box.
The other option would be a pistol safe that is bolted into the trunk area of your car. I know you said that you're a federal employee, but do they search your car before you get into the parking area? Even when I worked at Raytheon in Tucson if they did search your car (they usually only did this if you didn't have a Raytheon badge) they really didn't look that closely, and would not have noticed a pistol safe if even a small amount of thought went into concealing it. It's only for a couple of weeks, so I really wouldn't be too worried about getting caught unless they thoroughly search your car.
As to your other guns, I still think finding a friend with room in their safe is probably one of your better bets if the sister and brother don't work out. After that, maybe makeing some sort of arrangement with an FFL, or shooting club (if you're a member) might be possible. If you're planning on buying another gun or three, and a bunch ammo or accessories you could probably convince a dealer to store your existing collection for that time. Or find a gunsmithing shop and have them do a little work on each gun such that they'll need at least a couple weeks just to do all that. That way you'll get them safely stored and get some improvments/maintenance done at the same time.