Make sure the valve for the line that supplies the antifreeze is closed, Our old camper would suck air if not.
open ONE cold water line, the farthest is best if you can figure it out. once you have a steady flow, open the next one, and next till ALL cold is flowing steady.
Repeat the same for the hot, it will take longer for the tank to fill, As Cableman said you can open the blowoff on the WH. Make sure your bypass valves are correct.
2015 Shamrock 21DK
2005 Silverado Ext cab
Nights camped 2016..... not enough