Technician reported finding excessive debris in fuel tanks when tanks were opened to determine cause of numerous fuel indication and imbalance write-ups.

Narrative

I was called out to acft XXXX because of a fuel imbalance see log X and Y. I called Maintenance Controller about the history of the fuel indication.. He said sump the acft to make sure there is no water in fuel tanks then apply MEL XXXXY...After numerous write ups on the fuel system blanking out and qty indication being wrong.. All fuel tanks where opened up and thats where all the problems start to show..see logs that where created....LOG ZLOG ALOG BLOG CLOG DLOG YLOG ELOG FLOG XLOG GLOG HLOG ILOG JLOG KLOG LLOG MLOG NJust too many logs to record.. There's more to add.The fuel qty failed on the test flight out of ZZZ Day 0 see log A is where all this started...With all the trash that was left in the fuel tanks we are very lucky that we did not have inflight shut-down...........Cause: There was an unbelivable amount of trash;(large white absorbent towels; large metal funnel gallon jug) materials found inside fuel tank ... Keep in mind that this acft was inducted into heavy check @ ZZZ .. Why was this not discovered by QC before giving ok to close the fuel tank.... How can someone give ok to close? .. I just can't beleive that is even possible!! It seems that a ok to close the fuel tanks was given without looking inside!!!! ..;.we must follow AMM requirements when fuel tanks are open....this is a inspection buy back to verify the fuel tanks are clean and empty before closing up....not sure how all this trash was left in the fuel tank.