Buy a used RV and the batteries need to be tested. Assssssssumtions lead to nightmare camping and trips. This is where it pays gold dividends to look in the yellow pages...find a genuine battery shop.
Ring Ring, Hello you are a battery shop, right? Do you have a variable adjustable battery carbon pile load tester? No? Thank you (click).
When you find a shop that does have a carbon pile load tester, ask them if they can adjust specific gravity to TROPICAL BLEND. Use that term exactly. If they answer (huh? whuzzat?) it's "thank you, goodbye" time. This is a B.S. proofing question that separates the phonies from a real battery shop.
If your search gets frustrating, call a GOLF COURSE. Ask to speak to the "PRO". When the Pro comes online, ask them where they purchase their batteries. They are not going to buy from a fly by night distributor.
Take the batteries in, tell them you want them charged and load, plus gravity tested. Ask for numbers of test results. They may decline because they are overly busy, but asking is used as a tool. They will then determine you are dead serious.
A good battery shop will immediately look for the thermal imprint on the battery lid and they should shake their head in doubt if the battery is 3 years old or more. You should do the same. Few people even know how to take care of their batteries never mind follow up and do it. Expect a three year old battery to fail the test. The battery shop that handles sales to a golf course will not have garbage for it's RV battery inventory. Even if a battery costs ten dollars more than Costco or Wally it's worth it. You have an organization that will take care of you the right way.
Pay attention CAREFULLY to the advice other forum members are offering to you. This is a sophisticated crowd, and when they offer strongly worded advice it is usually right on the mark.
Standard RV converters are a battery seller's best friend. You need to PURCHASE something to fill the last 15% of battery capacity. Purchase and use a hydrometer to BS proof machinery and advice. When the hydrometer says "full", the deal is done. Hint: Landyachts extensive testing of manual power supplies is a gold mine. Don't worry about buying something strange to you. Buy a MeanWell just like his, and the folks on this forum will guide you through the process of installing it and making it work in terminology a 10-year old can understand.
Your "problem" should never had existed to begin with. Not at those temperatures. It's time to hit-the-trail and get things ready for next year. Camping should be trouble-free. Fun. No hassles at all. You can make this happen or choose to remain frustrated - your call.