How to make sure the house ends up in my name in the end? Took over payments for my parents and just want a document to show it will be mine when paid

You are wise to begin asking questions now.  There are several difficult issues.  The easiest solution to the concern would be for your dad and step-mom to simply convey the property to you subject to the mortgage --- but they can't do that for the following reason.  The first issue arises from the fact that there is a mortgage on the property.  It is almost certain that there is a "due on sale" clause in the mortgage.  That means the transfer of any interest in the property by your dad and step-mom to you would be a violation of the mortgage terms and would give the mortgage holder a basis for foreclosing -- unless you could obtain a written consent from the lender to allow the transfer.  Getting such consent is unlikely but, if the lender is a local bank, it might be worth a try if your dad and step-mom are willing to make the conveyance.  It is likely that even a contract to convey the property in the future would be deemed by the courts of your state as a violation of the "due on sale" clause.  Any other approach (short of an outright conveyance) would be uncertain, but may be all that you can reasonably get.  For example, your dad and step-mom could have wills prepared whereby they leave the property to you.  However, a will can be changed by either of them at their whim.  We don't know whether their deed is prepared in such a way that, if either of them dies, the title shifts entirely to the survivor.  If they have such joint ownership, your dad's death would leave the title entirely to your step-mom even if he has a will saying you get the property.  And if your step-mom dies without a will, her property (which would then include your home) would go to her heirs.  You are not an heir to her.  I recommend that you see a good real estate attorney in your area and ask him/her to research the title to your home to see exactly how the title is vested presently and to then advise you on what you can and should do to better protect your interests.  The cost of a limited title examination and a couple of conferences should be modest.