Gasoline in your fresh water system. That is one big OH OH!! Care to share??
Change 100% of the lines and tanks. Gas is very toxic with nasty long term health problems if consumed. It can not be 100% cleaned from plastic tanks or lines as the surface is not smooth when examined under a microscope and they absorb the gas to some extent.
The acceptable levels of gasoline in potable water are measured in parts per billion. Two table-spoons of gasoline is enough to contaminate a 20 ft X 40 Ft pool to unsafe levels. Don't take chances with your families health. (for those that say "prove my numbers" there is an old post where the calculations were shown, go ahead and look it up)
As for the unaccessable portions of the existing water lines, abandon them in place and re-route the new lines around obstacles.
Good Luck