I had a similar problem with my freshwater tank on a 1992 Fleetwood Caribou. The tank was located under the step up to the overhead. I was able to take the step off to look for the location of the leak. It turned out that the leak was on one of the corners of the tank, but it was inaccessible to fix. At the time, all the information I could find online and at the local RV dealer said that polyethylene could not be patched or repaired. I had no choice but to replace the tank, which was not cheap. The only tank that could be used to replace the old one was also one-third less in capacity.
If you have not located the leak, you might try this. With the camper on a slope towards the back, start filling the tank a little at a time and watch for signs of the water running back. The level at which the water shows up will determine where to look. If water shows quickly, look at connections to the pump and drain as well as lower corners. Mine showed up at half full, which led to finding it at one of the corners as it was an odd shaped tank. Hope this helps.