Bagful, as one word with a single letter 'l', has been in use since the 1300s to mean 'as much as a bag' - of one sort or another - 'will contain.' Thus, bagfuls is perfectly correct when used in that sense, as in, "Would you deliver three bagfuls of coal?' (Yes, of course, one could just say 'three bags' as well.) On the other hand, if you asked for 'three bags full of coal', the coal-merchant might think you imagined he'd be likely to cheat you by delivering three bags only partially filled! Similarly, 'I have bags full of stuff' also suggests that each bag is filled to capacity.
A key point is that 'bag fulls' as two words and with two letters 'l' is a non-starter.