Can I evict my son''s girlfriend from his trailer I have POA as he''s in jail

Interesting situation!  What does your son want you to do?  Your PA imposes a duty on you to sign his name and to otherwise perform such acts as are for his benefit.  If he wants the girlfriend to stay in the trailer, then I don't think you should attempt to evict her.  If he give you the authority to evict her under the P/A, then I think you can do so.  Get your authority in writing!  Since you can't (in my opinion) use the P/A to evict her without your son's authority, and since you don't own the ground, you do not appear to have any legal basis to evict her.  You are not without a remedy, however.  You have no duty to continue to pay the ground rent.  If you stop doing that, the girlfriend may resume those payments.  If so, the mobile home is not threatened, and you are relieved of the burden of paying.  If, however, she does not pay the ground rent, I will assume the owner of the lot will at some point force removal of the mobile home.  If that happens, the ownership of the trailer should not be threatened - only the cost of removal.  (Is it more costly to move the mobile home or to pay the ground rent?)  And, even if it is removed, the girlfriend may follow it -- particularly if your son wants her to occupy it.  If you haven't already done so, find out whether your son wants the girlfriend to stay, or if he is willing to allow you to evict.