Did you remove the backflow prevention balls, or just raise the filler necks? My Seismic 4116 still has the same filling problems after raising the necks as high as I could, about 4". I never found the balls. I am now planning to build a 'L' adapter out of aluminum tubing. Funny thing, Fuel transferred to the generator tank with the pump will flow at full volume. At the truck stop a half hour for 29 gallons in the front tank and an other hour filling the rear auxiliary tanks 47 gallons.
We bought our Seismic in Gulfport Ms from Dads Camper Outlet and were pulling it back to our home in Alamogordo NM approximately 1500 miles. We were having turbo problems on the Big Blacks Beast the '15 Ram 3500 drw, so we knew it would take at least 3 days maybe 4 days of Wal-Mart camping.
With the generator running 24-7 it would take a lot gas (or so l thought) so I filled both tanks. It only used the 29 gallons in the generator tank running almost 6 days.
It was a good thing I didn't need to transfer fuel because the pump was plumbed backwards. See my post elsewhere.