I agree with Gary, the little vent hose if it is clogged that bad, replace it. Getting the crud out is going to be complex and that crud will keep contaminating the water supply in the fresh tank.
A heads up on the larger fill hose, they do sell them at RV parts places and even on Amazon. You need to look for a "potable water" hose on the big hose and smaller hose needs to be safety rated from drinking water. It can be hard finding potable water vinyl tubing. It is made, but the normal average hardware store lumber yard does not have the potable water rated type.
The other heads up is to expand on what Gray said about the big hose sealing hard. The fresh tank (white plastic) is made from polyethene to get the potable water rating. That plastic does not like to have much of anything stick to it. I too use silicone on the hose, but try and not over do it or the hose will pop off the tank or the fill spout when you tighten the clamp fitting. Ideally you can find a smooth/smoother bore hose as some brands of the ribbed ones are worse then others to seal.
The silicone you want to use should be food grade as it is touching the water when it is cured. Do not use any ole type of silicone. If you search for "aquarium silicone" most of those brands (check to make sure) are food grade silicone as what is toxic to fish is also toxic to humans. Also, most of these are only food grade after they cure. When it is wet, there are cautions, but once cured for the proper length of time they can meet the food grade specs. I know, confusing.
I agree, you do not want a leak in the fill hose as water seeping in the camper is bad news. You will also need new butyl sealing tape to seal the fill spout flange back to the siding of the camper when you put it all back together.
PS, while there are limited places on a camper you should/can use silicone, like on the water hose, it should "not" be used to seal the siding or roof for water leaks. Silicone is not good or lasting in those applications. There are other better long term caulks made for that.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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