I don't think the tank is your problem...
My first guess is that residual odor from permeated hoses or a spill when the hoses were changed has attached itself to the area where the tank is. My next guess is trapped water in the bilge in that area...that can make a boat smell like a sewer in summer heat.So before you start cutting out fiberglass, open up the v-berth and liberally spray everything in sight--and not in sight--with K.O. Don't rinse...let it dry...and leave it open so that plenty of fresh air can get circulate in it...leave it open for a full day--or even till you come back next weekend. If that doesn't cure it, start looking for any place in that area where water could have accumulated. If none of the above helps, we'll explore replacing your tank.