J VIN Prime (All) and Hybrid (Some) all got a Yuasa flooded battery made in Japan. But its not your average Interstate. It a valve regulated battery that is typically sealed and under a little pressure. The results in the recombination of small amounts of Hydrogen and Oxygen being recycled to water. Now, LARGE amounts of oxygen and hydrogen under pressure is called an explosion, so the battery is referred to as a "Valve regulated" FLA (Flooded lead acid). Higher pressures will vent. It has a vent tube as well that goes through the floor of the trunk. The Yuasa battery is top end and probably will last a long time. It does have threaded caps with a o-ring, IIRC but you can indeed check and fill the cells. I believe it does have a sort of "sight gauge" too.
Like the Prius, US made RAV4 get a AGM battery. As other posters noted, Prius battery life tends to be a little better than your average vehicle. But the problem for a Prius (and Prime/Hybrid) is that there is NO warning when the battery is dying. A normal ICE engine with a starter "lets you know" the battery is failing in that you might hear it turning over more slowly. So you know you have a problem. Not so with Prius and RAV Hybrid/Prime. The battery just powers up the computer. The computer tells the traction battery to start the engine via M1 IIRC.
Solution: Every aging Prius or RAV4 Hybrid should have a jump pack. The one from Costco is fine. Just about any of them is fine. It only has to get the computer up and running. I have several of them in my life, one for each Prius and now one quite large one for my RAV4. Why large? Well, I am at that point in life where I have not a single conventional ICE engine left. I can not jump a friend or stranded motorist with ANY of my vehicles.
Amazon Jump Starter
I have used this just once so far, but it was for a diesel tractor that normally takes a pretty robust Group 35 battery. It turned it over and she started. It was 15 deg F out.
Whatever you get, try to make sure that it has a "boost" function. Normally these jump starters auto sense if you have hooked up your starter backwards and REFUSE to engage. However, if the battery is truly dead, say below 7v or so, the jump starter pack will not "see" it and will refuse to engage. That's where the "boost" button matters. You can manually tell it to engage.