I am preserving samples for both dna and rna for later research. stool, prostate fluid, urine, skin and nails all have rna?

Stability: I'm not even gonna ask WHY you're doing this, but unlike DNA, RNA is highly unstable, and tremendous effort must be put into extracting it and storing it. If you make cDNA from the RNA, that's a lot more stable. I would not expect any intact RNA in stool or urine. Skin cells do have RNA. Nails are made of protein, are not cellular, and would not contain RNA or DNA. Most cells have DNA (RBC don't).