I don't think I would ever be comfortable leaving 20 of my guns behind in the TH. I think I would take a few at a time and just leave the rest with my son and have him send and receive as I needed different things.
If you are anything like me most of those guns are just gonna sit and get shot once or twice a year... I have two long guns that I get 90 plus percent of my big game rifle hunting (I prefer the bow) done with and two shotguns I use regularly. The rest are for fun...
One storage option would be a heavy duty job box which you could leave in the back of the truck or in the garage... Just keep the guns in bags or buildout some shelves in the box...
Another thought is to build a safe or stack on box into/under the couch.