When I finally located my low point drains
I thought the same thing. Then I recalled the outside shower on the other side of the trailer. All that is needed then is a basin of some sort.
From my observation if you had a dollar for every one who "showered" with the outside shower you'd probably starve. But if you had a dollar for every one who used it to clean fish you'd be pretty well off.
It was an option Sweet Wife insisted on. Since 2008 we have never used it. Not once. Maybe I should take up fishing?
My drains are next to the entry door. If I wanted an outside sink under the awning, I think you are right. Put some shut off valves on the low point drains and rig some hoses to a sink faucet. Easy. The hard part as you say is finding a portable basin.
Until then I guess the shower and a bucket or dishpan will do. But those shut off valves on the drains would make winterizing that much easier.