Hi all. Been a fan of this website for a few years. Owned the K100RS 10 years. What a great bike. However..........
If you ride it locally around town the rpm remains at 800 - 850, even if it gets warm. However if you have sat on the motorway for some time and pull off to refuel the rpm is around the 1200 mark and remains steady at that. Once restarted after a period of 20 mins we are back to 800 - 850 again?  My guess is it has a slight air leak which opens up when hot, this weakens the mixture and raises the rpm on idle. 
I really don't want to start adjusting the fuel air screws but would like to know is this just another K100 thing?

Blacknails. Devon. England

Just stay of the motor way  
No seriously, has it done this for the 10 years you've had it or start recently?
My first thought would be the idle screw is set to high. Meaning set the idle when warm and use the first step on the "choke" around town till engine warms up. I have several Ks but one 88 k100lt take much longer to warm up enough to idle smooth.

Its always done it. Never bothered to sort it as its not a problem. Would like to find out what is going on. Never used the hi idle (Choke Lever) in all the years i have had it. Always started well even when left for a while. 

Blacknails

Check your vacuum pipes and also the vacuum caps. Very common issue.
Check thottle position sensor. Also very common.
Check you dont have something fouling or restricting the throttle from closing. Not uncommon especially if the clips on the throttle bodies have been replaced.

I never use the choke lever either but in cold I do have to hold the throttle open a little to get it to start. Its the same thing, all the choke does is advance the throttle slightly.

The fact that you don't have to do that suggests you are either running with the choke slightly on or possibly the throttle is not returning fully to rest which is the same thing. When the engine warms it then revs too high.