The first order of business is to put in a roof vent. Second is a solar panel. Third is some elementary electrical wiring - don't worry, this business has become so large (thank #vanlife) there are tons of plug & play kits available. Lacking that, there is now a very nice stable of qualified techs who do this for a living. Ok, so now you have a basic, yet very functional 12 volt solar system, complete with monitor, converter, battery (pack), etc.
I shared your entire post with my wife, and it was very intriguing. We are pretty handy (that comes with home ownership, remodeling, maintenance), and I have also done some automotive repair to include major engine work.
Let me ask you: on the roof vent, we are talking cutting the metal with an angle grinder, correct? Is the vent simply a vent with an electric motor for ventilation? If I had a camper van, I would want air conditioning, and I assume the factor air would only be sufficient for the front of the unit. Are there kits for an A/C unit for the back that would run on a Onan generator I could install? I have even heard of RVs running in lithium batteries, charged by solar power, and using a Soft Start kit.
https://www.gonewiththewynns.com/product/air-conditioning-off-grid
See the two videos associated with that link. Not sure if something like this is possible on smaller camper van.
Again, I love your idea. I can just see my wife designing the basic paneling (need to get my ShopSmith up and running).
Putting in a vent is super easy - just make sure you have the appropriate jig to cut straight & square. Any std RV vent (https://www.etrailer.com/RV-Vents-and-Fans/Fantastic-Vent/FV801208.html) will have a 1" fascia lap (this is the border that gets sealed and covered) anyway to create a nice, seamless look both inside/out. And yes, get one with a built in fan - some have nice auto thermostat features - that run off the 12VDC system.
As for A/C, now we're talking 120VAC - really, a whole different ball of wax than 12VDC. First of all, wiring of course much heavier, but you also run into the issue of dealing with minimum code requirements. If you want to do it right, that necessitates maybe having having a licensed contractor do the work. For example, while 12VDC uses standard automotive fuses, 120VAC requires household panel fuses. The level of complexity just keeps climbing if you factor in battery demands, and then a converter unit to recharge from shore power (20/30/50 amp).
Conclusion: unless you really, Really, REALLY need coach A/C, forget the 120VAC system. However, if A/C is a baseline requirement, then you're going to need to budget having a "real" 120VAC electrical wiring system installed for you.
As for generators, don't even think about a built in Onan that feeds off the main fuel line; that's crazy talk. Rather, just get either the Honda or Yamaha 2000 (peak watt, constant is 1600). These are the super quiet 'portable' 55 lbs models - if it's still too heavy, spend the money on having a HD trailer hitch box installed to keep it outside. BTW, the 2000 watt models cannot run the std RV A/C units that take around 14 amps (14a x 120v = 1680 watts). You can go up to a 3000 watt generator, but they are no longer portable.
Lastly, definitely spring for Li batteries. They are pretty expensive, but it's how you can keep a battery pack indoors without venting. With std lead acid or AGM, they have to be outside or in a exterior panel with venting.
I guess my basic advice would be to take it step by step. ID the absolute must have items and go from there. You might be surprised with what you can get away with. My solid must haves would be: vent (fan equipped), solar array (min 300-400 watts), Li battery pack (200ah min)/controller, roof rack with built in ladder, 5-10 gal water tanks (1 fresh, 1 gray), 12VDC water pump, simple counter/catch basin, marine toilet and 2000w generator.