I've had the same or similar leak in my 2000 Magna twice. It was at tthe connection of the corrugated drain hose from the sink to the hard plumbing in the bay just behind the slide. The first time the tech took off the small plexiglas panel you can see when looking directly at the bottom of the slide, not underneath in the bay. He then cut a whole bunch of wire ties to gain some flexibility in the corrugated drain hose. There is a panel, at least on my coach, in the bay just behind the slide. The panel is the length of the bay, about 8 inches or so high and at the top. It will be facing you. Remove the screws in that panel and remove it.
You will see where the corrugated hose connects to the hard plumbing for the coach. The problem I had both times was that the hose pulled out of the hard pipe. The first time for the repair the tech added a small extension of hard pipe and shoved the corrugate hose farther into the pipe and used a liberal amount of sealant. This repair lasted about 6 years. The second time, and at a different service facility, it was repaired the tech added more hard pipe and a nipple to go inside the corrugated hose and sealant between the nipple and corrugated hose. Then a hose clamp to hold the hose in place. I think this last repair will last.
If you repair by the second method you may not have to access the area above the plexiglas shield. There may be enough corrugated hose extending into the bay behind the slide. This will keep you from having to cut a bunch of wire ties and then replacing them after the repair.
Pete - Full Timing
2000 Country Coach Magna 40' Indulgence, CAT C10, #5892
2019 Ford Ranger XLT