Be careful not to totally close it off. As the water sloshes going down the road, it has to have somewhere to relieve pressure. If there is not a air vent, the pressure can cause water lines to burst in the LQ! My LQ has a small air vent above the screwed on fill cap. When I fill it up, I loose a little coming out the driveway, not really that much. Have you called the conversion company that did your LQ and asked them about it? Maybe your air vent is plugged.

 I have seen folks with some type of air vent deal they put on their outside spigot under the gooseneck. They travel with the pump off and spigot open just a little to relieve excessive air buildup in the tank.

One of my LQs has the twist type, marine drain plug that inserts below the vent hole:

http://www.basspro.com/webapp/wcs/stores/servlet/Product_10151_-1_10001_45245_225003005_225000000_225003000?hvarAID=shopping_googleproductads&om_mmc=shopping_googleproductads&affcode_c=17kw2359705&SST=43d26f23-b62f-76c9-cb14-0000345a9523

Thanks for  the link.   I live within 20 miles from salt water,  I should be able to access  a expanding plug close to home.