What TWP said. Venting is key in having water flow to the tank. Depending on your layout you could have a stack vent, through the roof or an air admittance vent with a one way valve.
Look under your cabinet for a "sweep T fitting" which should have the air admittance valve mounted to it.
The last option and one that is commonly used on the outdoor kitchens is a "grate seal", one way valve due to no p-trap for gasses. If you have these in your unit on the drains then these are very slow. Might be a new thing Jayco is going to in plumbing. See if you have a P-trap under the suspect sinks. If not then you most likely have this type setup.
2014 375 BHFS Eagle Premier
2014 Ram 3500 Longhorn DRW CC
6.7 CTD, Aisin, 4.10's