The Best New Battery Chargers For Your Van 

To verify, test out your 'max consumption' prior to buying, to size things up. I'm using a much less expensive (~$140) Victron 12 to 24V (adjustable up to 30V) 20A DC/DC converter for ~420W into the D2 or other solar charge controllers. Get/use a decent DC clamp meter to see what kind of amperage is comes off the alternator when you're powering as much possible, simultaneously. Add that number up for a 'current worst case consumption' figure, add ~50A for something like one of these, and give yourself at least a good 30% buffer between that and the alternator's rated amperage and you should be good to go. Additional power loads while driving likely. Most bigger vehicles should be alright running these. Maybe try to avoid AC usage at the same time, but for more specifics.. look for alternator info on amperage in parts catalogs / manuals for your particular model vehicle//alternator. Even sedans can have 175A alternators, so putting 30-50A should still give it headroom without running tons of stuff in your vehicle.

Check your factory alternator, ProMasters come with different amparage alternators, I think 180A or 220A. Mine came with the 220A and there's no issue running 40A DC-DC charger. I personally think it's a good idea to use DC-DC chargers in which the amparage can be controlled. For example, my DC-DC charger gives the option of selecting either 20A or 40A or turning off entirely. Yes, there's a lot of wiring labor involved but I think it is almost critical if you don't have a high average alternator. My DC-DC charger is from Renogy, and I don't think other affordable DC-DC chargers have these features--Ah, and Renogy DC-DC chargers have active cooling (fans that kick in when the unit gets hot) making it very desirable in enhancing efficiency. I intend to upgrade to 350A alternator and dedicate 80A of that to charging my solar batteries--I live in the PNW where the sun rarely shines in winter. Btw, I only run my DC-DC unit when the vehicle is in motion to not stress the alternator. Hope this helped!

There is a rating capacity on your alternator. that will tell you if you if it can handle the heavy additional load.. He does mention you can upgrade your alternator if it can't handle the additional load

Where? These Orion XS 50 amp 98% efficiency 12-12V DC chargers are new and don't really seem 'out there' for purchase yet. They're not exactly cheap either, but they're still cheaper than that Ecoflow $600. To its credit, though, the ecoflow does come with fairly nice cables though, it seems. If I see it deeply discounted to under $300, I might get it for that reverse charging capability alone. Super useful with a companion 12V LFP for expanding a Delta 2 or similar.

But wouldn't this result in more gasoline/diesel consumption ? Which is already so high in consumption... which is also very expensive ... I think keeping it to a minimum and just emergency / leftover charging if solar is not enough ?