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I know this is an old post, but the question it raises comes up a lot on other sites so a couple of us set out to answer this question by using our meters to take some measurements. What I did was use my Fluke clamp meter to measure amps and my standard Fluke meter with alligator clips on the charge lead in the camper to simultaneously measure voltage over a variety of conditions. Now this is with a 2012 Ford Super Duty F350 with a 6.2 liter gasser for power. I have no idea if these findings apply to GM or Dodge products.
First off, at the truck battery I found 14.1 VDC across all conditions. Voltage at the truck batteries did not change nor did engine speed increase amp output to the camper. We tested that at idle, 1,000, 2,000, and 3,000 RPM.
Secondly, with no load on the charge lead other than the parasitic load from the propane detector and a small digital voltage monitor (.5 amp total) that same 14.1 voltage is also measured at the distribution panel. Batteries were disconnected in this trail so they not being charged.
Third, with batteries at 12.4 VDC at the start and online with the charge lead, I measured 13.25 VDC and 4.6 amps from the charge lead to the batteries. Note: as soon as any significant draw is put on the charge lead from the truck voltage drops, however, the charge lead will still charge the batteries as long as demand is not excessive.
Fourth, with the batteries now off-line so only loads on the distribution panel are measured the question is what happens when the fridge is running on the DC setting as would be the case with the all electric refrigerators and the 3-ways set on DC.
Turning our Dometic 3-way fridge on using the DC setting, amp draw jumps to 12.5 and voltage drops to 11.50 VDC (not enough to charge the battery while the refrigerator is on DC). Add the furnace the mix and the amp draw jumps to 14 amps and voltage drops to 11.35 VDC.
Now results similar to these are also showing up on bench testing suggesting if high amp draw appliances are in the plans, solar or a dedicated charge lead through higher ga. wire may be worth consideration. The truck alternator is not likely able to both supply the power demands of the refrigerator and charge the batteries at the same time.
If anyone has done similar testing, it would be helpful to post those findings to increase the size of the database.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC