If you were under the UPC the minimum slope would be 1/4" per foot for 3" pipe. If by Mono-slab, you mean a post tensioned floor, then you cannot do ANYTHING to it except tunnel underneath. Which would be almost impossible for a 92' distance. Since there are sags in the line, measuring at the beginning and end of the line tells you NOTHING about the specific slope of the various portions of the pipe. About your only option is to find a "construction attorney" and see if your problem falls under the "undetectable failures" classification.