DC to DC charger

I have a DC-DC charger in my RV to charge from the truck. It's connected to the battery, not the alternator. the max draw from the battery is 150% of the charger capacity (ours is 60 charger, so 90 at the battery. We have dual truck betteries and like 397A alternator in F350 Super Duty) I cant tell you for certain you'd be fine, but I certainly would feel comfortable installing a 30A charger with a 270A alternator

You’ll be fine. The DC.DC charger can pull as much as 50% more than it’s rated output. Size the input cables according to the mfgr’s recommendation for the length of the circuit.

The charger should be placed as close as possible to the battery it’s charging, not the truck starting battery.

:):)

The charger should be a close to the batteries as possible. If you are connecting to the 7 way trailer harness those are usually fused for 40A - 50Awhich is fine. No need to over size that wire.

I have a 50 amp on my 2019 Chevy with a 270 amp alternator charging four Trojan T-105s and have had no issues.

WE run a victron 12-12-30 amp DC to DC charger, no problems.