Emptying wastewater tank

Our old VW campervan did not have a built-in wastewater tank - it was a case of a bucket outside and when it was full we took it over to the area for wastewater.  We now have a Peugeot Symbol with a built-in wastewater tank with a tap to drain the water out.  However, we like frequenting certificated locations who rarely have the hard-standing with the drain in the middle for this purpose.  What are we meant to do?  We asked a fellow camper at our last site and he suggested just opening the tap in the field and letting it out or opening the tap and letting it come out as you drive along - neither of which we would be comfortable with.  Help.