The vent may be plugged or kinked, causing it to blow back when the tank becomes pressurized. If the vent is working okay you shouldn't get blow back when filling with a normal hose on a residential system until it's full.
Can you see the tank and verify that it's actually only partially full? The level sensors or convenience panel lights may not be working correctly. On our 3100 the fresh water tank is under the bed, accessible by lifting the bottom of the underbed storage compartment. I don't know how to access it on your model.
PS Be sure to sanitize the tank and the entire water system (except the water heater) before you use the tank. Keep the water heater on bypass so the bleach doesn't get into the water heater. I'd flush out the water heater separately, too, just to be conservative. There's no telling when it was last flushed, if ever. Minerals and other stuff builds up in it.