Its been years since I've done a RB CG, but you can use a vehicle crew in place of an infantry crew to man the gun. This will depend on how the Refit Phase works regarding vehicles and their crews, which I don't remember. I do have a cloudy recollection that some of the RGs coming in with 1-2-7 crews. Generally speaking, crews refer to the both the vehicular 1-2-? and the infantry 2-2-? variety and there are no penalties for a 1-2-7 vehicular crew to man a Gun.
I did see in your post you have no extra crews available, but you may have been referring to the 2-2-8 infantry variety. If you have an extra 1-2-7 crew available, use it to man the Gun and therefore setup HIP and Emplaced. If not, might be time to request an RG with the 1-2-7 crews for subsequent CG days.
Good Luck!
Colin