Downside to combo might be wiring where you are forced into hard wiring, pass-through 120, and maybe sub-panels and confusion wrt the two legs in a 50a rig. But some folks prefer that in a MH.
Downside to separate might be if you want a higher amp charger than that 55amper to reduce gen time for recharging.
Inverter/chargers often come with 100a or 150a chargers, but that is rare among straight chargers. It is true you can get an inverter and a separate 100amp charger (eg, PowerMax PM4B-100) for less money ( perhaps a lot less) than the combo.
The combo would likely have some fancy settings options that you don't usually get otherwise if getting separate components