The generator starter needs quite a bit of current--mine takes somewhere around 90A according to my panel ammeter, on average, with considerable variation depending on how cold the temperature is, how close to up to speed the generator starter is turning, the phase of the moon, etc. You would have a hard time finding a switching power supply that would consistently provide sufficient current to start the generator without going into overcurrent shutdown mode or blowing an output fuse from the initial surge, and if you did find one it would be very much overkill for all the other uses.
You may be able to start the generator without a house battery by starting the main engine (which should engage the circuit to charge the house battery from the vehicle electrical system) and then starting the generator from the chassis battery. It may or may not work, depending on the exact design and robustness of the isolator and interconnection. If you have an emergency start switch for the main engine it stands a reasonable chance of working.
For other uses, I would think that ca. 25A or more would be easily sufficient.
If you do any camping on the road at all with this vehicle, a house battery is pretty much a necessity. I myself would not get rid of it unless I was also letting the registration and insurance lapse and hence deciding it's not going to be mobile anymore, but that's just me.
