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Have you considered raising compression? My 66 F100 got right at 20mpg with the 4spd/granny. Mostly this is due to compression. Probably the easier would be decking the head, but I've never dealt with a dual fuel system and not sure about compression and LPG.
Another thought is if you can run a smaller alternator. Not sure what it's running currently, but something smaller might create a bit less pull.
And while I think your bed cover came out great, have you considered the same shape, but removing the tailgate and bring the slope to the bottom of the bed? While you would get some drag from the bed sides, it will reduce dead air behind it. If you make it a hatch, your bed is still accessible. However, if you need all of your bed capacity, then it wouldn't be very helpful.
Something else that can also help, but not sure it's application for LPG is installing an oxygen sensor and gauge. On a gas motor, it will give you an idea of how rich or lean you are running... on an LPG engine... no idea.
Don't know if any of that is helpful, but thought I'd offer it up.
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