Forgive me for being insensitive, but there seems to be two issues here:
#1 is the sister & brother in law taking advantage of your father. I think the other posters recommending contacting Adult Protective Services if you are unable or unwilling to enter the premises and see what's going on is a excellent one.
#2 the loan. Your father signed it - worst case scenario is it gets repossessed, your father's credit is fouled up...but what difference does that make at this point? Can't get blood from a rock, yes it's a crap show & not a situation anyone wants to find themselves in, but them taking the RV back - will that in any way change your fathers life? He's 90, and his income is secure. It's not like he needs to buy a car or another house in the next 5-10 years I presume? Does he need excellent credit at this point in life?
Sorry, I sometimes take a overly practical view of things. If you can't or won't step in and get involved then you should accept that things are going to go even more sideways.