I used 2x4 lumber as cross braces and I only used 2 over the length of 44". The key to the this repair was using a very healthy amount of PL construction adhesive. I find this adhesive really made everything solid.
In the next day or two, I will try and grab a couple more pictures for you.
2010 Jayco JayFeather 213 EXP
2016 Ford F-150, 4x4, 3.5L ECOBOOST, MaxTow, Supercrew, 6.5FT, 3.55 axle