Engine will only start with AUX Battery Start

Battery cable connectors and posts on the batteries need to be clean and tight to make a good connection. Ground cables connectors also need to be clean and tight. Take connections apart and use a wire brush and battery terminal cleaner device available from auto supply store. Go buy a cheap digital multi-meter from Harbor Freight, etc. Read the instructions. Set it on the 20 volt DC (direct current) scale. Put the red probe on the positive battery terminal and black one on the negative terminal. With the truck engine running, you should read around 14 volts at the starting battery. Engine off you should read around 13 volts. When your rig is on shore power you should read 13+ volts at you house battery(s), if so, your converter/charger is working. NOTE: The converter/charger does not charge the starting battery on most rigs. Check battery electrolyte levels bi-monthly, add distilled water to just cover the plates inside the batteries using a "turkey baster" type battery filler from the auto supply store. Batteries and connections need periodic maintenance and checking using a voltmeter or they will let you down when you need them. Get familiar with 12 volt DC and 120 volt AC systems in your rig including circuit breakers and fuse locations, it'll come in handy on the road.