Thanks guys. I decided to flush the system anyway and put the correct Mopar stuff in there. Upon draining the system I discovered a partially clogged radiator.
At first it looked like rust brown material, but after some flushes and water coming out transparent, there's some small white and brown/black pebbles comming out. My guess is they are mineral deposits, calcium and silicates.
I tried filling the radiator with vinegar overnight but it did almost nothing, still looks as clogged as before.
I don't really have time or money to install a new radiator, I've been trying to clean the system so I can put the correct Mopar MS-9769 and forget about it, but that stuff is expensive and I don't want to fill the system with a clogged radiator. I will try a citric acid flush tomorrow, some Mercedes Sprinter guys recommend it on their forums, and see what comes out. Any advice for or against it are welcome.
On another note, I tried finding the radiator drain plug as per Service Manual instructions, but it's non existent, so I just removed the bottom radiator hose. I have spent so much time trying to find the engine block coolant drain plug but I can't, it's anoying to do flushes because I have to get the engine up to temperature every time so the thermostat opens and I can get the water out of the block. I found a 17mm plug at intake/front side of the engine, but once removal only a couple of drops of oil came out and nothing else. Did you guys ever found the engine block drain plug?