The house brand prices seem appropriate to me. Typical RV plumbing antifreeze is much cheaper than automotive antifreeze concentrate, due to less expensive components. A couple of the Prestone products are an exception to this pricing; perhaps there is a reason for that, but unless I saw a technical difference that was of benefit, I wouldn't pay the premium.
The
expensive Prestone is, unlike the
inexpensive Prestone, listed as non-flammable. This suggests that the extra money may get you
propylene glycol instead of alcohol, but that's only a guess. The
Material Safety Data Sheets would likely confirm or correct this.
Okay, just checked... that's it. The MSDS also show that the Prestone-branded products are made by Recochem, just like the house brands.