RE: can i convert port and starboard fiberglass water tanks 100 gal each to diesel fuel tanks
FRP tanks have been used in industry for storage of all kinds of organic liquid*compounds for years - since the 1960s in fact. Yes, there are types of resins and glass that are better than others, but most will work quite fine. In older cheaply made FRP tanks diesel can weep or leak in*places water would not, especially around the tank fitting*connections.
Could you then convert your your old diesel tanks to water by inserting a rubber bladder? Rubber can leave a funny taste but for showers etc it should work great.