My batteries won't charge from being hooked to my truck

[72Blazerod]

I'm having a problem getting my battery to charge up when it's hooked to my truck. When I went to raise the trailer after having been plugged into 120v all winter, it was dead. I had to plug the 120v back in to raise the tongue. This past week, before we went camping, I charged up the 12v battery (a cheap car battery because that's all I had at my disposal). We drove for 6 hours and the battery was fine for about 3 days. I decided to charge up the battery by hooking the trailer up to my truck. My brother-in-law brought over his battery/load tester and told me that there was no charge getting to the battery (it was at 10.5V and not moving). He thinks it's my truck (new to me 2006 GMC 2500 Duramax crew cab) because in his 2010 Ford F150 he had to install a fuse and a relay (that were in his glove box) to get his battery to charge. I think that it may be my trailer. What colour are the wires that lead from the 7 pin connector to the converter/charger? (I have a 1986 29' Sovereign, rear beds). I wouldn't be surprised if one of he previous owners disconnected the converter from the 7 pin connector lead. Sorry for being so long winded.

Thanks,

Rod

[JackMac]

Hi Rod, Looking at the 7-pin plug on your truck, the pin at the 1 o'clock position is the +12 volt supply going to the trailer. Check it with a volt meter ---- +12 volts should be present at all times, even with the engine off. A fuse must be installed in the truck to make that happen. In my 2001 Silverado it is a 40 amp fuse. I'm not sure what ampere fuse is in the newer trucks, however I am sure one is there because I had to put one in my friend's 2010 Silverado to get power to his trailer. It is located in the fuse box under the hood on the left side.