I own a RTV 900 also, purchased it new in Feb 2007 ....
The temp guages on these machines are not the best in the world. When replacing the guage, I would get a guage that has the numbers and actually tells one what the temp is . Some machines show the same temp, some running just off the rail, and some getting close to the red, but still, it will be in the safe zone. Just something to think about .
Now, the fan .... These fans cycle. Be careful.... but I would run a " hot " wire direct and see if it kicks in. Don't know how much you have run the machine. But first thing you better do is get a guage on the machine to tell if the temp of the machine is in the area that the fan needs to be a kicking on .
Don't guess at the temp, go ahead and take that step and fix the guage first.