My experience is that you need a "filling tube" that attaches to the end of the hose you're using to fill the tank. It goes ~ 10 or 12 inches into the tank fill receptacle and allows the venting tube to work properly. Camco sells one for ~$5 https://www.etrailer.com/RV-Plumbing...JCHxoCjk7w_wcB
but they are pretty easy to make from parts available at local hardware stores. I put a on/off valve on the one I made. As you are filling the tank you can tell by listening that the tank is nearly full.
RE: the question about filling the water heater. Filling the water fresh water tank will NOT alone fill the hot water tank. Once the fresh water tank is filled, close the hot water tank "by-pass" off and open the valves to and from the HW heater, turn the system pump on and open the "hot water" faucet on any of the sinks or the tub/shower and the hot water heater tank will fill. DO NOT turn the heater on until the tank is full!!!.