We just bought the 3231CK as well (not legacy) and got a medium firmness mattress from Brooklyn Bedding - they make them in RV King sizes. The 70x74 worked well for us. An inch or two of space at the headboard, and a little shorter than the stock mattress but it is so comfortable.
We are thrilled with it.
Link below!
https://brooklynbedding.com/products...g-70x74-medium
You could do the 70x80 as well, it just sticks out a bit more and you have less space to walk in the bedroom. I am tall but tend to curl up so the 70x74 has been fine for us.
We didn't even get the additional mattress topper, just the mattress and its wonderful. For extra comfort, you can add on the 1.5" topper - it is stitched on, so you cannot add it later (found this out the hard way), but we have really no need for it because we find the standard mattress without the topper supremely comfortable.
Our friends slept in it for a couple of days and loved it, so they are getting one for their RV as well.
We got a Predator 5000 genny, and were able to run both AC's on it (only for testing purposes). You can run them in tandem. We mounted some straps in the front cargo area and tied it down in there. Haven't needed to use it other than to test it yet. It's fairly quiet, fairly portable and a great price point. This is the Harbor Freight brand - tends to have very good reviews. If we did a lot of boondocking, I would consider two of them in tandem.
How are you finding the 3231 otherwise? We love ours! Had a few issues so far, but nothing we couldn't fix.