It is important to say that OEMs set limitations on their systems and they create their own systems with special Operating System images. My recommendation is to contact Dell and have you system re-imaged.
From our side, you can try the Generic driver we have for OEM systems. Here is the Intel® Rapid Storage Technology User guide. This document contains very detail information for users:
I have tried to install the RST driver from Intel's website, but as a result the system fails to boot. My suspicion is that there must have been a driver or a program booting from the now missing partitions, without which BIOS cannot handle the drives in raid mode, so i only see one of the two drives when setting up Windows. I would be more then thankful for a bootable app that would reinstall the missing partitions. I am afraid reading the manual you have suggested would not do it without the proper setup software.
Having quickly skimmed the manual it appears that this what I am missing: Rapid Storage Technology option ROM.