Had a 2013 3121 with the same issue. Piss poor design. I solved the draft and bug problem with pool noodles. Cut to wedge in the gap and notched around the slide rails. Putting the noodles in became a standard procedure for us, and it beat having a cold draft and bugs. We had several dirt dobbers and wasps make it inside as well, prior to the noodles.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.