First, some terms: series and parallel. They're important.
SERIES: Pair of real heavy 6v batteries that lasts a long time in your camper's 12v system. Double the voltage, 2 x 6 = 12, but the current (amp-hour ratings apply here) stays the same as a single battery (lots of folks are surprised by that last bit).
PARALLEL: Pair of 12v batteries crank the big engine in your van. Double the current of a single battery (cca here) but the voltage stays the same. These two wiring schemes are very useful but neither is magical. You can multiply either voltage or amperage by connecting things differently but never both.
Trojan T105 are the yardstick of 6v golf cart batteries. 225ah 20hr rating so figure 110ah usable power before recharge. Best to keep them above 50% for long life and under 20% is basically fatal. When selecting a charger, the charge rate should be between 10% and 13% of the battery’s 20-hour ah capacity. For example, with a 20-hour capacity rating of 225 ah use a charger rated between 23 and 30 amps (for multiple battery charging figure by ah rating of the entire bank), so double those numbers for a bank of 2 or simply put: 46-60 amp optimum charge for the bank of 2 auxiliary batteries. This is per Trojan's Tech Info. I say optimum because you can certainly go over that amperage if your system is well regulated to 14.8v daily charge (engine running or solar max) and 13.2v float charge and 15.5 equalization voltage. Again, this is direct from Trojan. These numbers are only exact at 80°F. Add or subtract .028v for every 10° below or above that, respectively. (If 70° add .028v and if 90° subtract .028v, etc)
Long story short, a well regulated system can't have too much power. It'll only push/pull (again, choose the term you like) as much amperage as cables (you need #1 or better) (good for 130 amps @ ~160F IIRC) and connections allow and regulating voltage precisely is what matters most. Undercharge = stratification and overcharge = boiling. Both very bad, especially with flooded/wet cells.
Many battery experts suggest operating wet types between 50 and 85% charge. This is best but requires frequent equalization, frequent watering, and more careful voltage regulation than most recreational users have time/patience for. I used these batteries in commercial applications (golf carts, manlifts, mining conveyors, etc) and we just charged each pair to 14.6v in use, to 13.2v during off hours/storage and then equalized seasonally or during other service, normally about 3x per year, and got EXCELLENT service from them, ~5 years avg in daily exercise with ZERO unexpected/unexplained failures under very un-battery-friendly conditions (cold, hot, high-vibration, damp, etc).
For alternator output to battery matching, I've always seen industry standards of battery bank total ah / 4 = alternator output in amps. This is common for boats and commercial applications so I've used it and never had a problem.
Your stock starting batteries + house batteries total probably about 325 ah. Reserve capacity of group 65 batteries is usually about 120 min @ 25a, so 50ah x 2 batteries [parallel] = 100ah stock + 225ah dual battery auxiliary bank [series], so 325 ah / 4 = 81.25 amp alternator, call it 90, is the minimum I'd want in that application. More is not worse but be advised these (truly) deep-cycle wet cell batteries with extremely heavy lead plates (that's why you're buyin them-the thick plates) charge slower than standard car batteries, MUCH slower than AGMs, and more current won't speed things up much, see below.
Recommended charge rates for a 80% discharged flooded-cell deep-cycle 6v battery is 10 hours @ 20 amps but for 50% it's reduced to 6hr @ 20a. This is per battery. Regular usage for me would be maximum discharge of 50% and would need 12hr @ 20a to replenish or 6hr @ 40a or (more realistically) 4hr @ 60a, or whatever's available as free current after all the other electrical loads are supplied. To extend battery life I'd hope to NEVER discharge them below 20% and on a daily camping/extended overland trip I'd keep it to 50% max discharge, this is to reduce charge times but also to keep batteries healthy longer and reduce the need to equalize them.
Correcting a 50% discharge on 1 T105 battery needs:
26. hr @ 5 amps
13. hr @ 10 amps
6.0 hr @ 20 amps
That's one battery. If you have 2 batteries you can either maintain amperage and double the time frame (probably more like 210% due to increased resistance in a series connection) or you can double the amperage and keep the time as-is (probably more like 110%, again due to the fact that all the charge current must flow completely through both batteries).
This answer is all about alternator(s) charging the batteries, a boondocking application so to speak, NOT alternator(s) powering high-amp-engine-running loads, like an ambulance application.
