On your dance floor bracing, Scamp
puts one piece of angle iron side to side in about the center of the floor area. This allows for a space for a grey water tank if you or a later owner decides to install one. On the taller 16'&19's some people make brackets to store their 12 gal. sewer totes behind the tank and center brace. Once a good floor is installed the frame is pretty solid. I do like the idea if adding outboard braces to help support the outer floor and shell.