Towable RVs do not come standard with an onboard generator unless they are of the toyhauler variety, nonetheless it is not too difficult to attain a quality portable inverter genny for your new TT. Compared to most Onans they sip fuel, a 5 gallon container of gas carried in the back of your truck could last days.
I see no reason why a TT can't be stored ready for the road, ours is stored in our side-yard always plugged in, fridge is on and fully stocked when not winterized. A few minutes to hitch up and away we go, our truck is dedicated to towing our rig and just sits in the garage fueled up and awaiting duty.
We love having an onboard Onan, pressing a single button to provide 120Vac to our rig as if plugged into shorepower. Unfortunately, Onan is way overkill 80% of the time. We purchased the Champion 3100 remote start inverter genny, like our Onan, a single press of a button and it easily powers our AC unit, even on eco-mode.