Hi RLC,
I know what you mean about the PICC line. I was so underweight by the time I got mine, they had a hard time getting a good vein. Luckily my arm was so numbed from the local, that I didn't feel them poking around as much as they actually were. It was only later than I saw some of the marks that I realized how much trouble it was.
One thing to caution you about, though, just in case you may have been given the wrong impression: you will not actually gain weight on TPN, but it will ensure you get the nutrients you need while you're system gets a rest. Hopefully this is the kickstart your body needs on the road to recovery.