To answer the original question, I think standard package that sportmobile sells now is 2 100 amp AGM batteries, this can be upgraded top a single 4D battery that would also be around 200 amps. Yes, you could put 2-4D batteries in parallel and get 400+ amps .
Things to check if moving to a larger battery bank, what is the capacity of your shore charger. I would want at least the capability to do 80 amps. How, well your alternator handles the additional load, will depend on the other power draws on the alternator. It would be very possible that you may need a larger alternator. In most cases it should be fine, but you may have some issues at idle.
Do you have solar, how many amps does the controller capable of and how many watts of panels do you have. You don't want to big of a battery bank that you can't get it recharged back to 100% SOC. That is one of the advantages of lithium and carbon foam agm, they can handle not being completely charged back to 100% .
Another carbon agm battery you may look at is Northstar Blue+ Batteries
The biggest problem with these are their sizes, as these are traditional used in telecom, so they are nonstandard sizes. But the big thing is they support PSOC like the firefly and lithium batteries. There price range is attractive and they are made in the USA.
As far as converting to lithium, some of that was covered in this thread http://www.sportsmobileforum.com/for...e-25017-2.html
One of the things to look at when going to lithium is the maximum output charge of the battery. This is usually determined by the capability of the FET's used in their BMS, and are the limiting factor. So if using heavy loads such as large inverter you may be better off with two 100 amp batteries versus one 200 amp battery