For sure, a portable power station can be the sole source for supplying all the electrical needs in a van build. Using one simplifies the knowledge required to build DIY. In addition, it saves space, has flexibility (can be removed and used outside the vehicle), and comes with a warranty. The biggest downside was cost, but that is changing.
Power stations have a few AC and DC ports, but beyond minimalist builds, the the location or number may be insufficient. Appliances that are permanently installed like a rooftop fan or diesel heater, are typically hard wired. For these applications, a busbar can be installed and there are power stations that can accommodate such a connection.
There are a growing number of manufacturers entering this niche, but if you want a large system, there are only a few choices. Bluetti and EcoFlow make power stations that allow for the creation of massive battery banks. The best choice depends on your priories. Bluetti makes external batteries that can be used standalone. EcoFlow excels when it comes to charging speed.
Small and medium size power stations are easy to charge. With large stations, the right size for a battery bank is primarily a function of charging preferences. Specifically, the length of time you want to be able to go without charging and the charging sources you want to use.
I wanted a battery bank large enough for me to be able to go at least 3 days with minimal charging inconvenience. I frequent cities and towns, I’m rarely stationary for more than a day or two, and don’t have access to RV hookups while on the road. Generators are extremely cost effective, but unacceptable in my habitat. Solar is unreliable for much of the year. My van is my primary vehicle and the main method of charging is via the cigarette port, it's trickle charge (100W) is sufficient to keep it topped off so that I don't need to plug it in routinely. When I do plug in, I want to get charged and get going. That is one of the primary reasons why I choose EcoFlow over Bluetti. My Delta Pro charges at 1800W when plugged into 120V AC (and 3400W when plugged into an EV charging station, a feature I've found surprisingly convenient on road trips). I've also used alternator charging via a dedicated inverter connected to the starter battery, with mixed results.