We have an 04 Allure. It was OEM X's 1 8D Chassis and X's 2 8D House - yep, for a total of X's 3 8D's in our battery bay.
I found that I had room in my battery compartment, to replace the X's 2 8D's, with X's 4 L16's. So we now have 800AH's of House Battery, which can of course yield 400 AH's down to 50% SOC. (When boon docking, we usually end up in the AM at 75% + or - 3%. And we do have the Samsung RF18, and assure you we do not try hard to conserve energy
Since your so close, I'd suggest a run down to Premier, and ask Jim Cooley if they can work you in for a changeover to the X's 4 L16's. (They fit in the same basic footprint of the 8D's, and the shop I worked with fabricated a simple hold down bracket for the four batteries. But IMP, simple strapping would be more then sufficient to hold them in place.)
As far a sourcing, I found form CC owner Tom (Now SOB Prevost), gives good advice as well as did a fair job of pricing. Link below.
Trans-Specialist - Your Source for Lifeline Batteries
You could also ask Premier to give you an estimate from their Lifeline source.
Now that being said, have you tried a few rounds of Conditioning the existing 8D Lifeline's. I found that due to some less then stellar software on the MS2812 (Set for Custom to do the new higher Float value for Lifeline's. But once it reaches fully charge, their logic does not kick back into Maintenance Mode until the batteries reach a certain lower voltage, and then it goes into Float Mode, for only 4 hours, before closing down again. The problem with this logic, is that the voltage they used to start back up maintenance, is for Wet Cells, not AGM's, so the batteries are down to about 30-35% before it kicked back into Float Mode, and 4 hours of Float is a drop in the bucket for 800AH's of House Batteries. These values were not customizable. And when the coach is is storage for up to 2-3 months at a time, with loads still on it, it did what is called 'Walking Down' the batteries.)
But back to Conditioning, I now do a 4 hour Conditioning run on my new bank of X's 4 L16's every 6-8 weeks, to keep the healthy - and shake off (Techie Word) any sulfation build up on the matts. You might try a few Conditioning cycles on your current bank of 8D's, and see if they restore enough to be useable.
Best of luck to you, have fun. And consider this to be my $.02 you can use for a new bank of House batteries