How can I fix my water system?

When I fill my water system from the Aquaroll, the water fills the hot water Truma tank first and then overflows from the tank. If I try to bleed the system by opening any tap, the water pours out of the hot water overflow and drains the system by sucking out all the water. No matter what I do, the water keeps draining from the hot water overflow and I cannot use the system. I have ended up having to isolate the hot water tank by blanking the pipe. The cold water works fine. HELP !!!!