you have to refill the portable tank, regardless
you will be better served, by filling both the rv tank and the portable
shut off the rv tank, use the portable, until empty or darn near
switch to RV tank, go fill the portable, then go back on portable
this will give you the most time on the portable, and the the most time until you have to move the RV for refill
trying to fill the rv from the portable won't work efficiently and you will only shorten the use time on the portable tank, and have to refill it sooner
