Interesting that you know him.. never thought the advice was narrow minded.. far from it, as far as I am aware, Sterling don't sell batteries, so no axe to grind ..I agree, Charles Sterling is the most narrow minded man I have ever spoken too. He is on his own with that opinion. There are several types of batteries out there and all have their own strengths and weaknesses.
Cost is a big factor
Got to agree with jim on this price wise, but the new agm batteries are good but pricey. Not a good idea to use with a high load inverter.There is no doubt in my mind that 'bang for buck' a flooded lead acid is the best choice and if you can afford them, buy deep cycle traction batteries, such as Trojan..
The second part of that quote is true (charging both batteries), but it's not the whole story.Below is a few lines from the link that lunarman gave. Makes for intresting reading. thanks to lunarman for the link.
If your battery is twice the age of the warranty, we strongly suggest you replace it. If the Manufacturer thought it would still perform at its best for longer, it would have a longer guarantee.
However, the primary reason we advise a Wet Acid battery, apart from saving a lot of money and charging more quickly,
is because in the majority of Motorhomes the Charger unit charges both Starter and Habitation Battery.