The generator starts off the house batteries. We also have a 2015 Winne Vista we'd bought new and had to replace the house batteries last season. The 12 volt "deep cycles" it comes with are not the optimal thing to use. I suspect your two house batteries are at end of life now. Go to Costco or Sams Club and get two 6 volt golf cart batteries or four if you are OK with spending a little more..but then you'll also need to pick up 3 more cables to complete the install. The 6 volts get wired in series rather than parallel. You will need one of these plates for each battery and some 3/16 threaded rod from Home Depot/Lowes. The 6 volts will sit against the outer lip of the battery tray, the hold down plate will run across the back top edge of the battery and the threaded rod will run at a 45 degree angle toward the outside of the RV (you have to drill holes in the tray for them). Even with just two 6 volts you will see a huge improvement in 12 volt power time for TVs, etc. If you have one of the larger 2015s with the 2K inverter, they will also let you do some limited microwaving at a rest stop without having to crank up the genny.
https://www.ebay.com/itm/Yamaha-Gas-Golf-Cart-Battery-Hold-Down-Plate-G1-to-G22-1978-UP-New-Parts/121680830113?epid=14024867329&hash=item1c54be1ea1:g:0UwAAOSwNSxVSTiz:sc:USPSPriority!89436!US!-1