Mike,
Before you re-invent the wheel and turn your G5RV into a loop, one
trick you can try is to change the length of the coax feed line. It
helps to have a few jumpers of different lengths and barrel connectors.
Add/remove jumpers in different combinations and you should see some
change in the swr on the bands. The trick is to find the length that
will let you tune a formerly untunable band, while not making some
other band untunable.
73, Steve KW3A