From my limited experience with Motorhome waste systems m/homes have a waste water tank which can be emptied periodically by opening the valve is yours like that? , the ones I've come across have been metal , but some are now plastic . the metal lever types which is the empty valve usually has a internal thread on the end , probably 25mm you would need to find a suitable adapter - see 1st photo. If its plastic ( see second photo) measure the outlet diameter and go to your local hardware store and buy a length of flexible plastic pipe with fits snugly over it and lead it to the drain . If it's clear plastic hose about 13mm diameter you could then cover it in the corrugated gray tubing (28.5mm dia)( to hide any bits ) which is normally used on caravans for the waste water to service pitch drain point. It may be a bit of experimentation to get the pipe to fit the end of the drain valve and a roll of duct tape may help - please post a photo of your drain lever.