I have never heard of the brand, i think it may be tough finding a manual. I personally would install an aftermarket temp sender and gauge, even if just to test. Hook it up out the back of the machine and give it a good run and see what its telling you. If it sits about right, its a bad sender.
There really isn't a lot to the cooling system. The most common problem is the radiator blocking up. You check everything else but the problem is still there. I like Ken's idea of wetting the radiator core. Run the machine then shut it down. Use a spray bottle to wet the cooling pipes of the radiator and check for evaporation. Pretty much all the pipes should dry up about the same rate.
Using a non-contact thermometer is better than the one you have on your DMM, you can quickly change location and get a reading. They are really handy! They also give an instant reading, you don't need to wait for the probe to warm up.