You just joined. ALWAYS post Year,Brand/Model of RV and Diesel or Gas. This helps to get you good answers. There is nothing really on a diesel to prevent overfilling. The Fuel Gauge has nothing to do with filling. You need to observe if you get a small leak when filling and not just when you reach close to full. It may be as simple as a fill hose loose or cracked from age(where the year info helps on answers). Some tanks and gauges the tank does NOT have to be dropped(diesel) if you have a Monaco or Holiday Rambler, to check hoses and fuel gauge sending unit. Doug