The quickest way to charge small devices like phones and laptops, just get a single deep cycle battery, stay away from batteries that list cca's since it is more a starter battery than a deepcycle,
You can just get short jumper cables and while you are driving charge the battery, just find a way to make sure the cables dont slip off the batteries while driving, yes this is a short term solution, you can then just drill a hole to extend the inverter cables into the battery compartment so you can use the inverter.
The alternator in your bus probably puts out 14.0 or 14.2 volts while driving and this is below the ~14.8 volts necessary to fully charge the deep cycle battery, not to mention the alternator is not a 3 stage charger.
I have a battery isolator separating my starter batteries and one of my solar banks, If I were to design it again I would just use a manual battery switch instead of a isolator, it is imperative that you keep track of your batteries water level and voltage level so you know how well they are charging and how deep you are discharging them.
my bus thread, https://www.skoolie.net/forums/showthread.php?t=8860&highlight=bubb