I have used washing detergents designed for gelcoat, (pick it up at a marine store) and I have also used Lemon Joy on both my boat and coach for years. Both have worked well and did not damage the gelcoat, at least as far as I know. Not sure about the spray nine, never have used it. A good coat of wax works wonders on making the bugs come off easy and also makes it much easier to wash the coach! I used a good marine wax with teflon and most of the dust and dirt wash off very easy.
TV, 2000 Dodge RAM 2500 Quad cab short bed, Cummins 5.9, Edge Comp, Gauges.
5ver, 2011 Palamino Sabre 31 RETS.