running till one is virtually empty, then taking the top off and removing what is left (say a gallon) unless it is visually very clean

Both tanks have a fuel filler cap just above the low spot, so we often stick a 3/8" tube in there and suck a few cups up with an outboard motor type squeeze pump. If it is not clean and water-free we pump more then dump it.

Of course, this works only when in a nice calm spot.