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.
Cheers
Paul