Most European motorhomes have tanks that are good for 3-4 days before they need emptying. Motorhome aires have facilities to empty and refill with water for a nominal fee, usual a couple of Euros. There is one in Santa Pola, two each in La Marina and San Fulgencio and also, heading northwards, aires at Alicante and Campello.
Hopefully they are responsible and not dumping where they are although motorhomers are a nomadic bunch and seldom stop at any one place for more than a few days before moving to a new location. Generally, if you choose to stay in one place for any length of time, you would have a caravan.