If you feel the RV window has shifted (from natural sag or from road bounce), is now binding / twisted and is now making its sliding system bind, then perhaps the window should simply be removed? Re-seal using fresh soft putty, re-install and this time, re-center the window. Thus, un-binding its sliding track system.
For a cool video on how to re-seal a window, surf:
This would be my 1st attempt to fix this sticky window. Especially if the window was sliding good on its day 1. Thus, proving its probably shifted and is now binding...