Your missing a trick....
You only need a 3.3V isolated power source (buck)
Connect the DC output to the -ve battery and feed the +v from the van to the -ve output of the (one or more in parallel) isolated buck units....
13.2V + 3.3V == 16.5V
There is a board on aliexpress that is a 35A buck from 18-36V.....
https://www.aliexpress.com/item/4000088930622.html
Use the boost controller you have to step up the 12V to say 36V and feed it into one of the 35A units, you can then charge the whole system at 35A. Add a second board in parallel on the 36V output to get around 60A charge rate (your alternater will likely reach it's limit way before then on idle).
The power/energy limit is the single boost converter 13.2V at 20A input = 264W in and output at 3.3V with loses (20%) of 211W which at 3.3V == 64A
Separate thought, charging from your engine is very inefficient overall and would likely cost around $1 per kWh
I had to charge from the truck a few times a while back (when I was on 24V with lead-acid) and even tried a 12V inverter - 230V - power supply route to get a higher current.
If your fixed with the van for a while, maybe swap the alternator for a 24V model (truck) and then rig up a CC/CV buck for the 12V battery (similar units to the one above) and then a buck only charge route.
Another option - remove the regulator in the alternator and draw out the higher AC or DC voltage..
Another option - create a really large coil the size of the van roof, then park under a HV line...