This is NOT a reason to stay away from the drain line tee idea, but... You must keep a perfect-sealing drain valve on that line! If the drain valve drips when your pump is not running, it'll be an air leak INTO the pump suction once the pump runs, and you'll be wondering why the system doesn't pressurize.
I wouldn't have dreamed this up. An OP on this forum replaced a pump, thinking it was the problem. Actually an RV shop replaced it, and system wouldn't pressurize. Very poor water flow. OP replaced the drain valve and the system returned to normal. New pump might've helped, might've had enough better suction to draw in more air and cause more problem, but the leaking drain was a major factor. I'd guess that if yours starts leaking, you may have to cap or plug it some way to keep your pump working till you can get another drain valve.
