My 2013 Forest River V-Cross Vibe had the same siphon issue with the fresh water tank. The overflow/vent fitting was no where near the top of the tank on the sidewall. I re-routed that hose so that it loops above the top of the tank. Since the tank is under the chassis & under the slide out, I had to extend it forward (along the street side on my rig) to get it above the top of the tank. Now no worries. The tanks are so shallow there is only a few inches between placement of the low point drain and the vent.
The foam at the edges/penetrations of the coroplast is just to fill voids to keep the critters out and not a big issue. Mostly hollow above the coroplast on mine.
I haven't had a slow draining issue.