Hi mikemaron,
Your 2011 Sundance no doubt has a "gravity fill" for the fresh tank. If the vent line is clear and a 4' tube in the fill hose doesn't help, there's probably a portion of the fill hose that's sagging. You might try a longer tube, but be careful not to force it in; I think it might be possible to break the fitting where it connects to the fresh tank.
The Anderson Valve mentioned by NP_Chief got rid of this problem by adding overflows to the fresh tank and routing city water (under pressure) into the fresh tank.
You can approximate what the Anderson Valve does by teeing into the PEX that goes between the fresh tank and the suction side of the water pump.
Here's a link to a Sharkbite press-on Tee that doesn't require crimping. The 90 degree leg of the tee would get a PEX line to your UDC (drill a hole). Add a
cutoff valve and a
garden hose fitting to the PEX where it comes into the UDC. The barb type fitting could either be crimped to the PEX with appropriate tools, or for this application, you could probably use a hose clamp.
When it's time to fill the tank, you open the valve and use your water hose to push pressurized city water into the tank.
BUT, if you overfill the tank with pressurized water, you can damage the tank. So you'll need a garden hose water meter inline where you connect the water hose. The meters, like
this one, are not expensive, but they're not terribly accurate either. So if your fresh tank holds 50 gallons, you probably don't want to put in more than 40. You will get some spitting through the vent as you fill.