Purpose of the overflow tube below fresh water tank?

I have two tubes coming out of the under belly. Why beats the hell of me. I put a shut off valve on both as water come out both when it get full ( Im guessing its full ) I close the valves when the tank is full otherwise it spits water as I travel

OKay, now I have it. I hadn't considered the weight and mass of water, that makes perfect sense. Water is a quite heavy and dense substance that we often take for granted!

I hadn't considered the effect it would have on an empty plastic tank, but it makes sense that after all the air is driven out that as the tank comes back to rest, water would also flush out.

I am sure my overflow comes out the top of the fresh tank, it's just odd that it's routed on a hose down next to the drain. All the fresh fills have that port next to the main hole for the overflow and mine is caulked over. Probably less chance of coming loose and causing a warranty claim.

My rig is a Cruise Lite 262BHXL from Forest River. Hoping not to start another chain/rant/flame, you can tell the amount of TIME they spend trying to make things not break through the warranty period, but not long term maintainable. That said, I mean who cares because that's the world we live in and part of the fun of owning one of these things is all the little fix ups you can do make it work better!

As Old-Bicuit stated, it's the hose that tells me when to shut off the water source! Sounds like yours is operating as designed/expected. As for your questions; ask away!

It's a vent allowing air in as water goes out.

IF that overflow tube actually goes into tank at bottom then it is a 'stand pipe' design overflow IE: pipe stands up in tank and level of tank must rise top of stand pipe before flowing out.

Similar to ones in residential toilet tanks

Overflows are used to prevent pressurizing tank should vent plug up, stops water from running out filler tube and lets you know "HEY tank is full" :B

If it is truly the over flow hose that you are looking at and not your low point drain, it comes out near the top off the tank and exits at the bottom of the tank. The purpose of the hose is that your tank does not expand too much and break the attach points. It is not uncommon to see water coming out of the over flow hose for several mins once it is full because the plastic tank will expand and bellow at the top from the force of the water entering the tank. Once the water is turned off the tank returns to its original position and continues to flow until the excess water is expelled. You should be able to see the top of the hose where it exits the tank.