You forgot to mention what I'll bet is a premium price for this "Marine" battery
Looks to me like it's an ordinary 75Ah battery
, similar to what you can buy for a lot less at WalMart.
I guess if your trailer might be submerged in salt water that the "Stainless steel" terminals might be helpful, but that too will add to the price.
Sounds to me that the 920 value, which may mean something, is just some hype to sell the battery to unsuspecting buyers that are expecting the capacity of two huge 8D diesel starting, batteries in a group 24 package.
Remember, anything that has the word "Marine" attached, or is sold in a boating supply store, will cost more, even if it's the same item sold in RV stores.