Well, you first need to see what will fit. The first step is to look at your existing batteries. Somewhere on the label you will see the numerals 24,27 or 31 most likely. These are the Group size. Look on this chart to get the dimensions of the group. If you don't see these numbers, try to match what you have to one of the groups.
Group SizeLxWxH (inches) LxWxH (cm)
Group 24 Batteries 10.25 x 6.8125 x 8.875 26 x 17.3 x 22.5
Group 27 Batteries 12.0625 x 6.8125 x 8.875 30.6 x 17.3 x 22.5
Group 31 Batteries 13 x 6.8125 x 9.4375 33 x 17.3 x 24
Group 34 Batteries 10.25 x 6.8125 x 7.875 26 x 17.3 x 20
Here is the link if the chart above is hard to read: https://www.batteryequivalents.com/b...ize-chart.html
Sometimes you can put bigger batteries in the space where your existing ones are. You will have to measure the space and compare to the dimensions above.
Then consider the type. I doubt if you need lithiums, so your choices are flooded cell lead acid (FLA) or AGMs. FLAs have filler caps and need periodic checking of the electrolyte level and water addition. AGMs do not have filler caps and require no periodic checking and water addition. That is what I chose for my MH.
I bought two Group 31 WindyNation batteries on Amazon (FLAs can't be shipped so Amazon doesn't sell them) for $385 for the pair and they have worked very well.