You can probably estimate the amperage going through the #6 wire by measuring the miliamps voltage drop across the 6' length of the #6 wire.
Put one lead on the battery terminal for the truck, and the other on the battery lead for the trailer. If you get a 0.001 volt drop and the ohms are 0.03, you can use math to estimate the amperage flow through that wire.
Chances are that amperage and milivolt drop across the wire will be about the same.
Most clamp on amp meters are only rated at AC amperage, because it influxes 60 times a second, and the meters can pick this up. DC clamp on meters are much more rare, and the Amprobe one that I bought cost $375 a few years back. Now I hear that Sears sells a clamp on DC meter that only costs about $85.
It looks like you used the right things to get this project done. It should quickly fill your battery. However know that while the engine is at idle, there is not a lot of oil flow (volume) to the upper end of the valve train. So while you might not think it will do any damage to idle the engine for a hour, it can. Ford says not to idle their diesel trucks, even while hitching up the truck to the trailer.
The gas trucks can handle more idling, but still it is not 'good' for them.
That said, you can plug in your truck to the trailer for say 10 minutes, and some of the truck's battery power will go into the trailer. Then a short run of the truck will provide max amperage from the alternator. 10 minutes at 130 amps will provide 2 amp hours per minute, or upwards of 20 amp hours in 10 minutes.
Your RV will consume 35 amp hours per day to just run the propane detector and CO detector, and refrigerator. Add lights to this, and it will be around 50 AH daily.
You might also consider a solar panel.
SunElec.comYou can get a 100 watt solar panel that will also produce about 30 amp hours per day in full sun. Two advantages is that the solar panels shade the roof, and help it not get so warm inside.
Have fun camping!
Fred.
