As for building large rovers and mobile bases, I recommend to build it in a way that it becomes an integral part of a rocket for easy construction of the lifting booster below, like this: But it also depends on how you big your rover would be. Also, you must build it first on the SPH for fine-tuning and symmetry, then move it to VAB for the booster. Oh - and the CoM must be exactly at the center of the mobile base (especially if you're planning to land it on its own power) And if you don't like using mainsails, you can always go clustering (they're much easier to do now).