Since you mention a 400W UPS system, I'm guessing you've found my previous posts on my experiences. If not, check out the links in this thread:
I'm not sure you're going to be able to squeak out 13-14 hours with a 200W load, at least not with the Xantrex unit I've had experience with. If you had TWO of the units, you could probably do it, but...
The things take a good 12-16 hours - at least - to fully recharge from a fully discharged state. If you're driving the 13-14 hours, I'm guessing by the time you get rolling the next morning, you won't have had enough time to fully charge them.
However... now that I think of it, you're probably going to transport them in a cooler, or some insulated thing... right? If that's the case, then the heat loss will be very minimal and you might just be able to do what you want. It just all depends on how "intermittent" that intermittent load actually is. Sounds like it's experiement time!