Off hand, I'd estimate that the costs of what you're proposing would probably double (or more) the cost of the basic trailer. All of those batteries, computers, custom software, sensors, etc. don't come cheap, especially when you factor in liability insurance against the inevitable lawsuits. You'd have to sell a lot of units to amortize the R&D costs to a reasonable level.

My boss's trailer looks pretty roomy from the pictures he has shown me. There are some new trailers that are very light compared to their predecessors. My only question about them is their durability - they seem somewhat flimsy. However, given that most of these trailers are probably not towed more than a few thousand miles a year, that may not be such a factor. However, that low usage works against developing a hybrid trailer, too.

I do wonder if hybrid electric technology might make some of the larger self-propelled RVs more fuel efficient. I'm thinking along the lines of a serial hybrid design like a diesel electric locomotive instead of the type of hybrid technology used in most hybrid cars. Some people live in those large RVs and travel a lot each year. If you could get one from the current 8-10 MPG up to 15 MPG, that would save a lot of money each year for an RV driven a lot, say 12,000 miles each year.

12,000 miles per year at 8 MPG = 1500 gallons of fuel. At $2 a gallon, that's $3000 a year.
12,000 miles per year at 15 MPG = 800 gallons of fuel. At $2 a gallon, that's $1600 a year.
Savings = $1400 per year, or $7000 over a 5 year period.