Is it legal to place a sign in the window of my legally parked vehicle telling people not to rent here at the apt complex I live in?
I can't think of why it would be illegal (so long as you don't make any false statements) and you have a valid reason for telling people of your opinion.
I am a Williamson County Divorce Lawyer and this question is outside of my normal practice area. You should consult with a local lawyer for a more in depth analysis of your situation.
Good Luck!