Hey Brit Rider welcome to the club
Having a reserve is when you can switch to spare batteries after the ones you are using have gone dead. So I would use the 24's as well but hook them seperatly from the 27's using a two or three way switch. I would also put in a battery isolator to help with the charging process.
You my still need to beef up your alternater too.
When you suspect a problem with your batteries or charging system always check with a volt meter to give you some idea what is wrong.
Battery voltage is around 12.6 to 12.8.Charging voltage is over the 12.8 and up to as high as 15 voltsDC.