If there is no room on the tongue I would recommend to make room by building a platform.
Assuming that is not possible I would have gone with a second 20 pound tank for a direct swap as needed.
Also consider a standard automatic regulator to connect both tanks at the same time. Yes one can sit on the ground while in use. Regulator has a green/red indicator to show a tank is empty. A pointer tells which tank to replace. Very standard RV item when two tanks come OEM. I recommend Cavagna brand (amazon). Get new pigtails also. One to fit existing and a longer one for the portable.
