two problems

[Pahaska]

Go to a competent RV, solar, or electrical shop and have a small inverter wired to to trailer battery. While traveling down the hiways, the inverter will not take any capacity from the battery and use only tow vehicle alternator power. The ac power is a gift, at no cost. Most refrigs use only 3-5 amps at 120vac.

3 to 5 amps at 120v requires input to the inverter of more than 30 to 50 amps. Remember, P=EI. Inverters are not 100% efficient. Granted, the power comes from the tow vehicle alternator while underway, but stop for gas or a meal and there will be a truly great sucking sound out of the battery.

Refers using 12vdc will use up any batteries quickly.

Not nearly as quickly as the same refrigerator operating on 120v. The 12v heater element is rated at a much lower wattage than the 120v element. The 12v element is meant to keep the box cool, not cool it down. The battery will go down a lot smore slowly than when running the 120v element through an inverter.

Just as the inverter will use tow vehicle alternator power, so will the 12v element.

[FrankR]

Thank you John for the voltage/resistence/amps reminder.
My life has been spent mostly as an electrical contractor.
I think you may take some of what I have posted too literally. I am not a degreed person, but have encountered many electrical challenges. Yep, I really like inverters.
Your challenges to my post.
The tow vehicle usually makes lots more amps than can possibly be used just driving down a freeway. The current draw of the refer is for a relatively short period of time. The draw is not 50 amps for many hours.
I believe the 12 volt dc connection will cool down a refer from scratch and is designed for this condition. Using this voltage would be the only good time to use 12 vdc.
All of the trailers that I have wired, are wired in such a fashion that when ignition is off, there is no connection to trailer from tow vehicle batteries. The inverter is not drawing any energy while tow vehicle is stopped with engine not operating. The ignition switch controls a relay that connects trailer to tow vehicle. The relay may also be on a seperate switch/control...
I love to exchange ideas.....

[Pahaska]

12v will not cool down a refrigerator in warm weather unless you have a week to wait. The120v element wattage is the highest, distantly followed by 12v wattage according to the manual in my Scamp which had a 6 cu ft 3-way Dometic refrigerator. When the AC element died in the Scamp, I tried cooling down the Scamp box on 12v and, believe me, it didn't do much good.

The 12v heating element is a low-wattage compromise meant only to maintain an already cold box while sparing the battery as much as possible. With an already cold box, 12v operation will maintain internal temperature in all but the hottest weather so long as the door is not opened a lot.

While your alternator has all the capacity you need (and more) to run the box on AC, it is not free. You pay in gas mileage for loads on the alternator. Probably cheaper thn buying propane, but not free. Alternator load does matter; one of the reasons auto makers went to halogen lights was to eke out the last bit of CAFE mileage by reducing the electrical load.

Granted, the 120v load is not continuous, but it can be very nearly so in mid summer in Texas. Even with tow vehicle battery isolation, stopping for any substantial period and forgetting to turn the refrigerator off 120v will suck down a trailer battery in a hurry.

[Pahaska]

I should have mentioned that the 12v heating element is not controlled by the thermostat. It is on continuously when the selector is turned to DC operation. One more indication that 12v operation is a sustaining mode, not a cooling mode.