I have done this twice. Technically, you can't transfer TPUs. You can transfer deposits, but only the entire deposit. You can have several 'combined deposits' in your account, each with a unique relation number.
To see the list of your individual (even combined) deposits, sign in to RCI; click on 'Manage Deposits'; click on 'Weeks You've Already Deposited'.
You will see a list of all your weeks on deposit, including combined weeks and leftover TPUs from exchanges. Click on the red cross to the left of each week and you will see additional info, including the relationship number. The relationship number is how you identify the week you wish to transfer
The form to do so is on the RCI website at
http://www.rci.com/docs/KnowledgeBase/Documents/en_US/RCI_Wks_Mem_trans_agrmt_1.pdf
and the transfer fee is $99. regardless of how many weeks you transfer. To transfer weeks, you fill out sections A, B and D. I crossed out section C when I did this so RCI wouldn't mistakenly transfer ownership of the week vs. spacebanked deposits.
Does anyone know if you can transfer or purchase TPU's from another RCI member or directly from RCI as you can do with RCI points?