I think it's a bad idea, especially because of your intended use. You could pretty easily find yourself with no battery power to do anything, be it start the main engine or start the generator. The starting battery is not a deep cycle battery and isn't particularly well suited for significant discharges; it's instead more optimized to supply brief, high currents such as the starter needs.
On many motorhomes, the same relay is used for the emergency start as for the battery isolator system, as was mentioned, and often this is controlled for the isolator system by the engine run power line from the key switch. If that's the case, you can combine them temporarily by putting the key in the run position without starting the engine. I sometimes do this when exercising the generator for the opposite reason--to try to keep up the starting battery charge during longer-term storage.
