Doc prescribed triamcinolone acetonide 0.1% cream for rash that is covering my stomach & back. he said cover my abdomen & back with it. is that safe?

Short term: Short term yes. If not working in short order stop it and re see your doctor. The issue is how much of your body surface needs to be covered. If a lot it may act like an oral steroid, with its potential side effects.

Yes, but what is it!: I would ask the question of what is being treated. The prescription is not off the mark, but may not be the permanent solution, but a quick fix! it should be safe, if used for a brief period of time!