generator won't start

Hello guys :Get the generator started ! After a day checking all connections thru the process of elimination ,everything normal , OK except teh generator won't start before Dropping the generator to work on starter I put a a jump cable from a fully charged battery direcly to generator even it shown get 12.8 V. It starts right away, even with the remote. Take the battery off it 't not start, bring the extra battery hook to the built-in it work again. Turn out my built in are shot but somehow still give me 12.8 for light,pumps ,jacks but not enough amps to start the generator.That 12 Volts Interstate white/green looking only two years old! I don't know how the previous owner took care of it ,when I got it do the normal thing. equalize charged it put in lot of distilled water ,it got weaker when run all the slides,jack set up but recharged ok .So get new batteries and some extra on the way. Thank you all .