My IR gun would pick up anything that would be noticeably hot to the touch and it does help to be closer.
When we talk of running a microwave we say to use four 6v because while the Ah's may be there, the internal resistance will keep you from using it with just two. Four will do it but at the amount of run time you are talking, I wonder if you run into the same issue.
Using the solar to assist the bank can help keep the bank up but small wiring between the bank and the inverter is adding resistance itself. You may want to see if those wires are getting warmer while you run it because as the heat builds up, so does the resistance.
I have had our MW run 12 minutes nuking four large taters and three more minutes wouldn't have been a issue. Then again we have a little more solar, 50% more bank in 12v AGM that puke when you nuke and much larger wiring everywhere.
