At the risk of being a smart-arse trying to spoil Bruce's day, when he's making such good progress..... no, go on, I'll be a smart-arse:
Adding diagonals to the framework to resist sideways loads is perfectly sensible for a house, but completely unnecessary for a trailer where the skin is bonded on properly.
Diagonals are added to houses because the 'skins' are not continuous and are not that well fixed-on. In the trailer, the skin itself/themselves act as the diagonals, just like they do in aircraft and in boats, neither of which have diagonals (at least in stressed-skin construction).
And you can't get an example of skins that are better bonded-on than Bruce's. How come you have a vacuum pump at home, Bruce, or are you using a shop vac? (I've never had the confidence to try that, as I assume it'll burn out the vac motor.)
Andrew