Most any marine store can supply you with one, or at least the pump to make one. They are used to wash the decks of larger boats - it's called a wash-deck pump for that reason. It will likely be a 12v pump, though.
Most any water pump with a decent "head" (suction) should do the job.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC