Kohler CH18 engine closure cover bolts keep loosening up

Are you torquing them in sequence to the proper spec?

I am afraid that short cuts in engine work don't cut it. Yes it is pain at times to remove an engine but it must be done, no way you can apply the RTV as needed and replace the cover without smearing it. Just like replacing the steering shaft on some Cub Cadets requires removing the rear fender pan so you can get the operator station apart.
 
First since these engines are assemble with RTV as the gasket the surface of the mating surface must be completely free of oil. Before applying the RTV the surface prep / accelerator must be applied. Then once the RTV is applied and the assemble is complete you must wait at 24 hrs before adding oil. If not done this way oil will prevent the RTV from fully curing. Oh it hold for a while but not as permanent as when done correctly.
 
As Roy said the bolts must be tighten in the sequence that is in the service manual and to the specified torque amount. Then if the bolts back out then applied a small amount of blue Loctite, not the red Loctite.

If you feel it is too loose then replace it. The governor will need to installed at the correct height.
Here is the instructions:

 
As for the bolts they shouldn't work loose but they do then use some blue Loctite. I am not at the engine so I can't what is going on. What Kohler uses for RTV on these is Permatex Ultra Black rtv which has the maximum oil resistance when properly applied.