Using Truck Alternator to Charge House Batteries

First, any tow vehicle with a factory tow package is going to have an upgraded alternator to handle the increased load experienced when the TT is plugged into the truck's connector. If you feel the need to connect your camper to the truck because the batteries are low, simply plug it in near the hitch, and it will charge them just fine.
Second, The way a vehicle's charging system works is, the vehicle actually runs off the battery for all it's electrical requirements, and while old fashioned generators had to run at higher rpm's before the charging started, today's alternators produce electricity at very low rpm's, so no need for high idle to charge the battery.
Purpose built generators provide nearly all their generated electricity for such things as battery recharging, which makes them much more efficient than your TV for recharging. Even at that, it takes several hours to recharge batteries to 100% with one.
hope this helps.

To charge two discharged 6v GC2's will be many hours until 100% SOC. You could probably get to 90% in less than 4 hrs..

When boondocking I connect a battery charger to my trailer batteries (2 x 12V in parallel) powered by a 400W inverter from a cig lighter in the truck for keeping the batteries topped off. We do not use an inverter to create 120V for anything in the trailer so we are very frugal with battery draw. A tea kettle is used to boil water for instant coffee, tea, & cocoa.

I now have a quiet 2KW generator the wife can use for her hair dryer rather than borrow power from a fellow camper in a motorhome so this winter will be different. I need 120v to inflate an airbed couch daily when guests bunk with us.

Other than putting one heck of a load on your alternator by asking it to do something it was not designed for, I don't think you are getting much accomplished. As the previous poster mentioned, @ idle not much charging is going on. I guess that in an emergency situation or in a pinch, it would be decent. But for something like this to be employed as habit, you are asking a lot from your truck's electrical system.

Also, someone who knows cars more than me might contend that connecting and disconnecting dissimilar electrical circuits is bad for the truck's computers. I don't know how much more robust computers in vehicles are now when compared to one 10 years ago, I do know that in the past, it was always frowned upon from the mechanics that I spoke with.

first of all , why do you need to use a keruig coffee maker ...have you ever used a 'percolator 'coffee maker on your gas stove ... thats what I use when I camp with out power...camping does not always require electricity...imop

at idle most charging systems can only produce 30 percent Max. you are mostly using the battery in the truck to run your coffee pot.

if you could run the truck engine at 1,500 to 2,000 then you would be charging your trailer batteries.

Some people do it. Don't see it causing much harm. I wouldn't idle my truck for much more than 25 minutes at a time though.