There is nothing wrong with fibreglass tanks. They are used in many applications today. However,fibreglassing over sheet metal does not seem to work well. What you get is a fibreglass/metal composite material that tends to delaminate and leak. You could build a replacement tank from scratch out of fibreglass but it would probably be cheaper and easier to buy a replacement tank or make one out of steel.
If I had this problem, I would move the tank out of the cab and use replacement tanks as suggested in other posts.
