Not likely that you have a switch, filling from either side goes into the same tank.
What does the fuel gauge say after adding 55 gallons?
Could be you were filling it too fast, creating foam as diesel tends to do, and that is what shut it off. Did you attempt to add more after it shut down?
My Winnebago would shut down, then I could slowly add 14 more gallons.
2017 Entegra Anthem 44B, Gold Ribbon, Ambience
2004 Winnebago Vectra 40AD (Sold)
1980 Hilary Tent (traded in on Winnebago)