http://2cw8eb1vmmgg3g5i7jzt6upo.wpengine.netdna-cdn.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdfAccording to Chapter 5 it should take:
10/100 x300 = 30/10amps = 3 +2hrs = 5 hours
But with solar you have the amps climb to mid-day values and then taper again as the sun gets lower in the afternoon. So hard to pick the amps number to use there.
It says full is 13.0v and 75% SOC (25% DOD) is 12.5v
Is the camper plugged into the truck when you are parked? If it is and the truck is a Chev, there will be a draw from the truck.