From what I can tell, the Natruell Fuel System is only available to convert gasoline engines. Prins mentions diesel engine systems on their Transport Market page but not on their CNG Products page. The only other universal natural gas conversion system for diesel-powered trucks I am aware of is the EcoDiesel System made by Technocarb. The LPG version has just been made commercially available and the CNG version should be available shortly. I expect that any CNG system would only be available to commercial installers for liability reasons.

The price of $7000 for a CNG conversion seems to be in the ballpark. CNG cylinders are more expensive than LPG tanks and there is more cost involved with components that withstand CNG's 3600 psi storage pressure.