The M119 engine tends to run hot in a W124 chassis like yours and mine. Mine will easily go to 105-110 in summer heat, in stop and go traffic, with AC on. Having said that, yours is too hot.
1. If you suspect a leak, the only way to really find it is to pressurize the system. You need to buy or rent a pressure tester. I bought one. They cost about $80 but I think it's worth it.
2. Do you have a new "radiator" cap? If not, get one.
3. How old is your water pump? Is it OE? Some aftermarket rebuilts are said to have smaller vanes so don't move as much water.
4. How are your aux fans running? Have you tested both low and high speed operation? In my case, getting them running again (by replacing a burned out resistor) got the temp down.
5. The other main mod is to "fix" the fan clutch so the mechanical fan is working 100% of the time instead of temperature-dependent mode. It is set at the factory to only couple when it sees about 107-109 degrees of AMBIENT AIR TEMP which means by the time it couples, the coolant temp is MUCH higher. Stu Ritter of the STAR magazine has a good fix. My tech modified his approach. I'll be happy to detail it for you if you are interested. This single mod has made more difference for me than any other single change I've made. Now, even in 100 F temps, coolant never exceeds 100 C in traffic.
6. Water Wetter helps a little but not much. You still have something wrong somewhere if you are seeing 115 C. On the other hand, do make sure you have NO MORE than 50% MB coolant. Seems to me that every time my coolant gets flushed, I have more like 60/40 coolant/water. If I take out some of the mix and add straight water to get more like 50/50, or even 45/55, the temp does come down a few degrees. You can check your mixture with a Prestone coolant tester.