Battery question, opinions needed

It’ll work for what you’re using it for, which is virtually nothing. It’ll also probably not last long since on in the pair already died (unless it lived an unusually short life for some reason).
The reason “not” to use start batteries in general is they don’t handle deeper discharges as well and if used like a typical deep cycle, won’t last near as long. Nothing else fundamentally incorrect about using it.
Just like deep cycles generally can’t handle the peak amp draws of a start battery when cranking a large engine over.

How much you save will depend on how long it lasts for “your” purposes. Worst case you’re only postponing another new battery a short time. Best case it might live there for years.

I don't see an issue with this - 12V is 12V. The battery just won't last as long, but it'll give you some time before needing to purchase a new one. I'd do it.

CC

Nothing wrong with what you are thinking about and that's exactly what I would do. Buy two new batteries for the truck and put the older but still good truck battery in the trailer. I've never had any problem returning a mismatched core for the refund of the core fees.

2 new bats for truck; save the good 12 volt for the rv.

I'd rely on the converter rather than buying a maintainer--unless you already own one.

go for it.

So you full time host, is for Oregon or other states. I will tell you that there are many times, as host we deal without power. Sometimes and hour or two sometimes several days. If I were you I would keep the dump the single battery and keep the the four pack. I am assuming the four pack consists of 6 volt cells.

Sounds like a plan to me.

One caution: you say "I never boondock" but you never know.

I was parked comfortable with full 50 amp when suddenly BANG No power No internet No _____ Thanfully I had tanked water.

Seems just past the drive for the campground the road curved left
The car went straight... Into one of those man made T-Shaped Trees (yup the parks power feed). and BANG.. No power

Thanks everyone for the very helpful input. The whole thing turned into a Murphy's law experience. I bought the new batteries online at the closest Walmart 80 miles away. Everything was confirmed for curbside pickup in 3 hrs. Somehow they screwed up the order so bad that my debit card got flagged for fraud. Bank said it was their fault. Took 2 hours to fix the card issue. Then walmart "said" they didn't have the batteries. Had to go to a Walmart an extra 25 miles away. So 210 miles round trip.

I was concerned that maybe an overcharging alternator might have caused the battery problem. But I couldn't test that until the new batteries went in. The battery install was easy. Fired it up. Check engine light came on. It was running fine but charging at over 15 volts. One of the trouble codes was a probable faulty ECM which would have been a $2,000+ dealer only repair.

It was late in the day. I decided to sleep on it. Started watching YouTube videos instead. Among other things, I learned that the ECM regulates the charge from the alternator. It will go over 15 volts if temp is freezing or below. It was 30° and dropping when I started it with new batteries. Eventually I came up with this extremely hopeful theory that the multiple trouble codes resulted mainly from the bad battery (it could have easily been the other way around). I couldn't get hold of my mechanic, so I deleted the codes and went for a long test drive the next day after it warmed up to 45°.

Apparently, the theory I derived from over a dozen videos was correct. The trouble codes didn't come back, charging was below 15 volts, and it all held up during a long test drive. Hopefully that all holds firm and we're off to our next park tomorrow. Yay.

Btw, if you don't have one, get a battery tester. Topdon makes some really good ones for under $60. I got mine on eBay for $23. It measures CCA, internal resistance, SOC, etc. It also tests your alternator and starter. The cheaper one I got assumes you have some knowledge. The midrange ones do the same thing but assume you're a novice. The expensive ones do pretty much the same thing, but they give a printout of results for your customers. Also, battery maintainers have gotten cheap. I got the Sigma 1 for under $30. Both of these devices are light-years ahead of anything I thought I'd ever own. Heck, the code reader I have works on every car manufactured since 1996. It's now about $15. I think I paid $50 when I bought it a few years back. These and other newer electronic tools make the RV life so much easier. All of mine together cost under $200 today. I easily saved more than that on just this problem.