I have a 2011 Road Warrior which is somewhat the same style. I had to change the duct going to the garage. It was twisted and had breaks in it. I changed it to an insulated type of heater duct and I get plenty of heat. Maybe not as much as I would prefer, but it's an improvement. I don't use a ceramic heater because when i'm not hauling my toys, my 3 year old plays back there. Just a precaution for us. I also insulated the bottom of the toy section with 1" foam insulation. Since I installed it I haven't tested it. We are going to Disney World for Memorial Weekend, so hopefully it should be cooler back there also. It doesn't get very cool back there either. Sorry for the long winded response.
Ivan