Six gallons in your fresh water tank probably isn't enough the reach the pump suction, but is enough to take several fill/drains to get it all rinsed out so water is drinkable again. You should've, possibly could've, pumped it directly from the jug, would've only used a gallon or so.
You didn't list your rv, some have a hose tied to pump in the convenience center for this purpose.
Be sure to pour some down all drains to fill the P traps & leave plenty in the toilet bowl to keep flush seal covered.
Danny & Linda
Former '13 Redwood FB owner