I'll just add what they told me about letting a sewer truck drain your black tanks.
I do a lot of sporting events where a residential sewer truck comes around to drain your tanks.
Most will take a bucket and put your sewer hose in it and add their big suction hose to the bucket. Their suction is higher than the sewer hose and it works just fine. It is frightening to watch the first time you see it done that way though!
One event the sewer truck guy had an adapter and was sucking it straight from the tank. Thank god my neighbor was the RV mobile repair on site.
He said NOT to allow them to drain the tanks. The suction from those commercial trucks will collapse an RV black tank.