Really need more info as to what you are wanting to do...
Please give model and year of your POPUP camper...
I suspect you want to add something like this to your POPUP. I don't know if you can buy a hinge-over assy or not. Almost sounds like you may have to build up what changes you want to do. The fold-over assy when folded over has to clear the push in tent beds ends when closing up the trailer for travel.
This is a photo of the fold-over sink assy for a 1994 Rockwood model 1910 POPUP from google search - This listing is for around $1700 from the Pensacola area..
Purchasing the sink would be the easy part... You will also need a fresh water tank with water pump of some fashion... COLEMAN sells a portable sink hookup as I recall from my tent camping days...
Roy Ken