Typically, banks require to be in first position. If you are sourcing funding from family and friends they'll want to be secured as well.
They can be secured with a second mortgage/trust deed or with an equity position or a combination of those two.
Make sure it's all in writing. Memories tend to get fuzzy when things don't go as planned.
Your bank will want proof of your down payment long before escrow closes.
Make sure you are crystal clear with your investors as to how they make money or how they could lose money. Anything else invites lawyers, etc.
If you have very little or no skin in the game and no experience in the MHP space, what are your investors really getting themselves into?
BTW, if you're getting 10% in your retirement account, what are your expectations for this project?
One more thing, you could lend some of your retirement money to an experienced operator in exchange for a preferential return.
Just a thought...
Keep us posted.