How does the oven get gas? On my RV range (and indeed most gas ranges I've seen), the incoming gas line connects to a manifold (via a regulator), and then the valves for the burners and the thermostatic valve for the oven all connect to the manifold. It almost sounds as though your unit has the stove top and the oven as two completely separate units with a tee or something in the incoming gas line...rather strange sounding to me, but I guess it's possible.
If they are actually two separate units, possibly assembled together with some screws or whatever, then it should be quite straightforward and safe to replace the stovetop with a smaller unit, assuming you can work out the physical mounting details.
Or maybe this isn't a gas oven at all, but an electrical convection or microwave or something? In that case, again they're basically separate and replacing the stovetop part should be fairly easy.
There are also combination microwave/(electric) convection ovens available, and it would be pretty straightforward to physically mount one below a cooktop, replacing the existing range entirely. Naturally an electric convection oven won't run without shore power or a generator.
