For 6 month storage I would not add either cetane or biocide additives. Especially if it is during cooler weather (algae growth slowed in cold weather). Make sure the tank is full, to avoid any moisture condensation from air cooling in a near empty tank.
If you are concerned with diesel gelling (cold weather, less than 10F) and you want to start the unit, make sure you have a winter diesel or non-bio diesel blend, or #1 diesel in the tank, or use an anti-gelling additive.
Hope this helps.
Towr: 2007 Country Coach Allure 470 - 37 Sunset Bay Cummins 400 ISL
Toad: 2007 Jeep Grand Cherokee (yes, it has a Hemi)