You sound a lot like us! We're a family of 3, and our son is 7.5 (but no step children that I know of
) We had a small hard-side A frame pop up that fit us well for a while and we started thinking about a bigger model, then thinking of a hybrid, then something with a rear bed slide like an Outback, then just finally decided to go for a Travel Trailer. We decided not to go for slides, though. One of our big requirements was bunk beds for DS and a friend if he brings one along, so we decided on the GS Visa 23BHD. The 23BHD has some of the biggest bunks out there, so he's definitely not squished. We had adult friends join us about a month ago (no kids weekend), and they fit fine in the bunks as well.
So, I'm not sure if you're interested in bunks or not...if so, then our model wouldn't have your slide that you're after, but it is a nice fit for a family of 3 or 4. The A/C has worked very well the times we've used it, and the heat and air are ducted in the Visas.