The safest way to do it is to slurp it out of a bucket.
Otherwise...lay the sewer hose out on the ground. Have the service man pick up a couple of feet of hose, insert his suction wand and pull his valve part way. It can't get enough suction to collapse a tank etc.
You pull your sewer valve part way. The trick is that you don't overrun the suction wand.
I was taught this by an old-timer in north Georgia; we could slurp out tanks in a minute or two complete with water rinse as possible. My cut was $10 per tank plus a freebie for me.