Can I gravity fill my on board tank?

Thanks for all of the help everyone. I just wanted to get some opinions on this subject. To me it would be obvious that I could gravity fill through the vent hole if I need to. So I called the manufacturer and they said "It's not a problem to fill through the vent hole and that I need to go slow so the air can vent out." The pressure fill inlet line is just the 1/2 inch pex line, which makes gravity fill extremely slow if at all possible due to the size. So I will be getting a transfer pump and there will be modification in the future when the warranty expires.