Eco Flow 3 Plus and alternator charger routing?

I have just bought the eco flow and charger, I plan to have the ecoflow at the rear of the van ( next to the 20v cigarette socket)but just looking at the best route for the charging cable, I know some have gone through the cupboards and shower room to the under seat main battery but that looks a bit tight between the shower room and diesel heater, and even tighter around the seat. I am also considering if i could drop it through the air vent in the rear and fasten to underside of body, perhaps coming up in the engine bay or indeed the battery bay under the seat( rubber Gromit etc) has anyone tried this route or is the cable too short?

We have a Bluetti PPS and B2B charger (mounted in electrical cupboard). From here to bench seat is quite straight forward, but most difficult bit is from bench seat to leisure battery. It's very tight here and I ended up with one wire under the floor and one above floor in a short piece of trunking. From B pillar to battery if pretty tight too.

I have exactly this setup. You need a 12V feed from the main battery + to the charger. I ran this throigh an exisiting drain hole in the bottom of the battery box and under the van then as yiu said back up through the vent tonthe alternator chstger. The alternator charger cable isn’t long enough for this so yiu have to do some cable splicing. I picked up the negative wart from the load tie down point in the space by the vent at the back.this all work jolly nicely.