Mr Squirrel's valve and bottom filler system is quite clever.
I have a portable filling rig that I take with me.  It is 25' of that
thin water hose like you connect an ice maker with.  I think it is
1/4" but don't remember if that is ID or OD.  It has a brass hose
connector on one end.  I stick a foot or so of the hose down the
filler tube and clip the hose to the open filler door with a clothes
pin or small clamp. It fills at just about 1 gallon/ minute.  I just
look at my watch or set a timer based on how much water I need.  I
usually don't carry more than I need because it's heavy.

I wonder if the people with slow filling tanks have clogged vents?

Edward