When filling with water, most of it dumps out the side.

Get one of these...

Turn the supply tap down to reduce the rate of flow.

I also use a dedicated short piece of hose to fill the fresh water tank. I have a spray bottle close at hand that is filled with diluted bleach solution. I also have a supply of wet wipes. I will spray and/or wipe down the end of the hose before using it.

I also use the wet wipes and bleach bottle for sanitation when I’m dumping tanks,.

I used an old hose (actually it was new, just sitting around a long time), for a washing machine and cut it in half. Both ends have the female end.

I then attach the 1/2 hose to my fill (garden hose), and simply insert it into the camper fresh water fill. The diameter of the washing machine hose is smaller than the fill hole hose, allowing air to escape around it (and the little vent also). I then never turn the spigot on full blast. I always let it fill slow.

Before, if I simply put the hose on the end of the fill port, the water would belch back-up. It became quite obvious the water was filling so fast it was causing a vacuum in the fill hose and literally throwing it up back up. When I slowed the water down, it fill just fine.

However, I eventually figured it out the tank was filling only 1/2 full when the water started overflowing the fill hole. Even running slow, it would cause a vacuum and then finally belch over (I though it was full, it wasn't).

That's when I came up with the idea of the washing machine hose cut in half add-on. It worked great! I shove the hose completely down the neck of the fill hole, run the water a little slower, and I've never had problems since. I keep the washing machine hose right in my pass through, keep it clean in a separate zip lock bag, and use it only for filling the fresh water tank.

The most simple solution to your problem is to fill the tank with a slower stream of water. Don't run the hose full blast. It will not back up and belch out that way.

And by the way, this same washing machine hose works absolutely marvelous for washing out the water heater. It fits perfect in the anode rode hole and still have room to let air in (vent), and when the spigot is turned on full force, it really blasts the inside of the water heater.

I have since added a garden hose turn off valve so I can shut the water off at the end of the hose and not get a bath running back and forth to the spigot, or having to lay the black washing machine hose on a dirty surface. This works fantastic. If you have a washing machine hose laying around, try it.

If you put the hose in too far, the hose itself could block the air from escaping. Secondly, I have a short piece of hose I use to put tightly against the vent hole and blow into it to remove any water that mlght be lying in there that may be blocking the air from escaping.

Maybe your fill line is kinked or the vent is plugged. Bad business shoving a hose in the filler anyway. That hose end is loaded with germs and who knows what else.

Obviously, the vent line needs cleared. Did you try running the water into the tank at slower flow?
If I gravity fill the tank on our 96 Southwind it will do the same thing unless I slow the flow. My vent is clear snd working normal.