I haven't specifically figured out my consumption yet, but I will be running a fridge, a few LED lights here and there and a 12v water pump; all powered by 2 X 100ah AGM Marine batteries.
Doesn't sound too different from what I got going myself, other than my batteries are in the vehicle itself (2×105Ah).
Since you plan to also have the batteries charge from your vehicle's alternator, I suspect you could get away with 110W of solar capacity (the 140W panel w/PWM controller) as long as you don't plan to be parked idle for more than a few days... You've got a pretty decent amount of battery capacity available. This should restore approximately the power your fridge consumes, though not much more than that (figure 110W × 5 hours of decent available sun per day = 550Wh, or maybe 7-8 hours of sun if you're able to follow the sun directly with your panel). It's imperative however that you run very heavy gauge wiring from your car's charging system (or from your isolator) out to the trailer though, as that's a rather long run of cable. Any resistance here
very quickly degrades your alternator's ability to fully charge the batteries (#1/0 AWG and using an Anderson power connector like what you usually see on a removable winch mount carrier is the minimum I'd recommend, like their SB-series connectors).
http://www.andersonpower.com/products/multipole-sb.html
Hopefully that helps... Sounds like a nice setup you're working on.