First off: While RealPlan does an okay job, AP still labels it as a "beta" function because it's still got some issues. You should set your expectations accordingly.
Second: To duplicate your results, what do you have set as the fuel burn for your plane?
Third: I don't know what you mean by "Class D and higher" for airspace avoidance. Did you check or uncheck Warning, MOA, Alert, Restricted, and Prohibited airspaces?
Finally: Are you creating two flight plans, one for your westbound Huntsville-to-Rockport flight, and a second one for the return Rockport-to-Huntsville flight? (If you're trying to put all that into a single flight plan, maybe that's part of the problem...see comment above about RealPlan still being in beta. The simpler you can make the problem for it to solve, to better your chances of getting a decent result.)
Using the 9.0 gph for my plane and the 25-gal capacity you specified, avoiding only B,C,D airspace but no others, at a cruise altitude of 4500', then when I enter a flight plan of MDQ - LBX (landing) - RKP, RealPlan gives me a route that climbs like a rocket to 6500' to clear HSV Class C, then plummets like a rock to land at M83 for fuel. The rest of the route is at 4500' and overflies CBM (Class C ceiling of 4200'), avoids Houston Class B, and doesn't come close to any others (BPT class D has a ceiling of 2500'). The other fuel stop is at ESF. (I didn't do the math to ensure the fuel stops make sense.)
Aside from the crazy climb/descent rates it requires, it meets the constraints I selected in RealPlan.