There's nowt wrong with connecting two 8D batteries in parallel. Are you thinking of adding a second 8D to an existing one?
If you're talking about all new batteries and have the space, why not use golf cart batteries? A pair of 8D's will give you roughly 480 amp hours of capacity. 4 golf cart batteries connected for 12V will give you 440 amp hours in a little less space and 6 of them would give you 660 amp hours in slightly more space than the pair of 8Ds. Golf cart batteries are designed for repeated heavy daily discharge.
Normally, deep cycle batteries would be re-charged either by an on-board generator or by plugging into shore power and and on-board charger . Unless you have a very high output alternator and/or are driving a lot of hours, an alternator won't adequately re-charge your batteries.
Can I assume that you have a travel trailer or 5th wheel rather than a motorhome? If so, what size alternator does your current tow vehicle have? Maybe someone else can provide more info on available alternator sizes.
Golf cart batteries are essentially the same as an 8D battery split in half. Putting two 6 volt golf cart batteries in series gives you approximately the same amount of power as you'll find in a single 8D battery. The advantage is the weight distribution - each golf cart battery weighs about half as much as the 8D, making them easier to lift in and out of the RV.