For the OP I installed my 4 Trojans today removing the two 12v. As you can see the battery box barely fits but is doable. Even if you only buy two 12V you should still buy the 4 battery box so you can easily upgrade. You can just build a filler to take up the extra space.
As mentioned above I purchased 4 Trojan T105 Plus 225 AH $125 Ea no tax and no core required ($500 total )
https://www.onlinebatterysale.com/store/
the Battery Box was purchased from All Battery Sales and Service $80
https://allbatterysalesandservice.co...atteries-gcq-g
I bought the 2" vent for the lid from Amazon $5
https://www.amazon.com/gp/product/B0...0?ie=UTF8&th=1
I Just ordered the Watering system for $79 so they will for the most part be Maintenance free as I will gravity feed the system.
https://www.rvupgradestore.com/Flow-...bg-u24v-4g.htm
Total Cost of new 4 new batteries and box $665
