First check the water inside the batteries (add only distilled water if low).
Then put them on a charger at home for 24hrs.
Then, take them to your local AutoZone or O'Reileys, etc and ask them to test the batteries. They will be able to tell you if the batteries are good or bad.
If they are bad, replace them. I recently acquired a 23' Class C RV to add to our other RVs. It had 2 batteries (Group 27s). They were behaving "weird". I would charge them, but they would quickly be dead. I put solar on them for a few days, and it looked like they were charged, but then after just a few hours they showed 10v. I brought them to AutoZone and they tested them and told me they were bad. So I went and bought two 6v GC batteries to replace them and now everything works exactly as expected. Summary: "Weird" battery behavior is often a sign that they are bad and need to be replaced.
If they are good, then it is time to put them back in your rig and start to figure out what is drawing them down.
I'm a huge believer in having a battery monitor. You can get inexpensive ones for $50 or less (I use the Aili 300amp one that can be found on Amazon for ~$45). Once installed, you will always know what is going on with your batteries! Well worth it.
When my RV sits with "nothing" on, my battery monitor tells me that it is drawing about .3 amps (...I think, I haven't looked lately). (My RV does not have a disconnect switch by the side door like many other RVs). That means that in a 24hr period it would use up about 8AH from my battery bank. So, over time, it would drain my house batteries.
If your battery monitor tells you that you are drawing more than 1 amp, you can start pulling 12v fuses from your panel until you find the culprit. Then you will be able to figure out if it is something normal or not.
One cheap solution (assuming your batteries are good) is to install this on your battery terminals:
Then, every time you leave your RV for days you can quickly disconnect your battery.
I personally HATE dead batteries...they drive me crazy. So to solve this, I added solar to all my RVs. 100-200 watts will make it virtually impossible for your batteries to be drawn down if it is parked for days/weeks. If you add 300-500w of solar (and you are a casual RVer without power needs beyond the ordinary), you will likely never have battery issues again even when using your RV to boondock.