12 volt DC thermostat

I have a motorhome that uses a 12VDC thermostat to control the AC/Heatpump and the propane gas furnace. The thermostat is a real piece of juink. It overshoots, undershoots and generally is not very good. It is, however, an electronic unit. I would like to replace it. I found a nice Honeywell, RTH7400D which looks like it might work, but the Honeywell Tech Supports says that it's not rated for use on 12VDC. However, it can be made to run on internal batteries and it does have relay contact outputs. So, I would think ..... But, anyway, does anyone know of a good AC/Heatpump/gas furnace thermostat that could operate a 12VDC system? BTW, I would ask this in the RV News Groups, but I think it is too HVAC specific for those NGs. Thanks,
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I'd give it a shot, you could hook the battery back up into the 12v system by matching the voltage (usually 9v dc) I'd imagine Radio shack has something to do 12v dc to 9v dc. I'd just run it on permanent battery backup.
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>I have a motorhome that uses a 12VDC thermostat to control the AC/Heatpump and the >propane gas furnace. The thermostat is a real piece of juink. It overshoots, undershoots >and generally is not very good. It is, however, an electronic unit. I would like to replace it. >I found a nice Honeywell, RTH7400D which looks like it might work, but the Honeywell >Tech Supports says that it's not rated for use on 12VDC. However, it can be made to run >on internal batteries and it does have relay contact outputs. So, I would think ..... But, >anyway, does anyone know of a good AC/Heatpump/gas furnace thermostat that could >operate a 12VDC system? BTW, I would ask this in the RV News Groups, but I think it is >too HVAC specific for those NGs. >Thanks,
Eric in North TX wrote:

I might just try that. Here's exactly what Honeywell said:
"Thank you for contacting Honeywell.
The RTH7400D is dual powered thermostat and it can be hardwired/ battery operated or both. It also has relays inside to turn on/off the heating/cooling system.
But we would still like to reiterate the fact that the RTH7400D is not compatible with 12 Vdc systems."
Sounds just like they never spec'd it for 12VDC.
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I'm not familiar with that particular tstat, but if it's battery operated, it should work. The batteries switch a relay.
I look at it like amping a starter on a vehicle. the ammeter only sees amps and has no idea if it's AC or DC.
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