We've been using toppers for 19 years and for us being fulltimers, we wouldn't be without them.  
There is no insulation in the top of the slides so having the topper will help keep you cooler in the summer as the heat will hit the topper 1st shading the top of the slides a bit.  Reverse in cold weather as it acts like a shield keeping the cold from directly going into the top of the slide.  
We too replaced the toppers and all of our awnings when we had our 5th wheel updated in 2010.  This is what works for us.  Hugs, Di