Low point drain valve question

A consumer grade pressure washer uses very little water flow. About a gallon/minute.

With that said, I interpret your question as you have shore water pressure and want to tap off the cold low point drain as a matter of convenience. If so, I'm confident you would get enough water pressure and flow to stay up with the washer.

Try it and let us know how well it works.

The valves have about half the size of your Pex lines. I doubt you could get enough volume thru one of those valves for a pressure washer. Are you planning on using the RV water pump and fresh tank as your water source? If so, another way to do what you want is to remove the check valve in the City water input and install a standard water hose manual cut off. When on Water pump that valve stays OFF and when on City Water you turn the valve ON. Doug

Depends. Look at the flow when you drain it. This is literally not a question we can answer. I've had campers trickle drain and others that run good.

But regardless, you have no pressure, so you won't get even close to adequate flow to feed your pressure washer. As in NO pressure.
Have to hook to the pressure side.