If it starts when the boost is used it's not the starter. Was the new battery FULLY charged before or after installation? Make sure the battery has a FULL charge. If the battery is fully charged you have a problem with the cables on the chassis battery. Sorry, I need to ask this question, you did install a 12 volt battery not a 6 volt battery for the chassis? If the chassis battery is a 12 volt check that cables are clean and tight. Check the ground cable where it attaches to the frame remove it and clean to bare metal.
1999 35 ft. Dolphin 5350, F53, Banks System, 5 Stars Tune, Air Lift Air Bags, Koni Shocks, Blue OX TruCenter, TigerTrak track bars F&R, Roadmaster 1-3/4" rear auxiliary sway bar, 2004 F450 Lariat Pickup 6.0 Diesel Crew Cab DRW, 4X4, GVWR 15,000, Front GAWR 6,000, Rear GAWR 11,000, GCWR 26,000,1994 36ft Avion 5er, GVWR 13,700, 2,740 Pin Weight.