First read this thread which includes a post I wrote discussing the various types of batteries- https://www.thorforums.com/forums/f8...ies-23130.html
You sound like you use your batteries for boondocking. In that case if they will fit, two golf cart batteries you can buy from Costco, Walmart, Batteries Plus, etc will cost about $250 for two and give you the best combination of cost and performance. Since they are 6V just wire them in series.
If they can't fit then there are very few true 12V deep cycle batteries that are traditional flooded cell types. Your best bet then is to install two Group 24, 27, or 31 AGMs. The higher numbers mean more amp hours but larger cases, so buy the biggest that will fit. These can be bought from Amazon probably the cheapest or Batteries Plus.
A small solar panel probably won't help you much. It will take a couple of 100 watt panels to do the job or maybe more. You can buy the panels from Amazon and Renogy or Victron make good controllers. Get an MPPT type.