Tap mixes warm into cold water

Yes. Bath faucet does the same thing.
Cold water comes out first when cold is turned on.
But when water pressure drops a little. . .the water pump turns on and the water stream turns warm (briefly).
And when the pump shuts off, the water stream reverts to cold.

And so it cycles cold/warm/cold/warm/cold.

I guess the other faucets do the same thing. Bathroom and shower? If they don't, then it would be an issue with the kitchen faucet.

The manual shows a check valve on the hot water outlet.
That check valve is to prevent anti-freeze or cold water from getting into the water heater according to the manual.

Could there be enough back pressure that hot water could siphon or be drawn into the cold water line?

My water heater appear to not have a check valve, only has the male-to-male adapter:

Okay, well I turned the bypass valve to the "other" direction and tried the water which started making choking noises.
I don't think that was the problem.

Tomorrow I will look at finding the check valve.
They build these truck campers really compact and there is nothing that is easy to get to.
Thanks atreis. That Suburban manual is a real good thing to have.

The check valve in the water heater isn't visible - it's built into the fitting.

Here's a picture of one:


Page 14 of this manual from Heartland describes how to replace it, including pictures:
Heartland Suburban Water Heater Troubleshooting Guide

There's other good info in there that might be useful as well.

I have do not have a check valve in my water heater just 3 valve water system and get NO mixing of hot/cold unless the 'bypass' valve is not completely shut.

The check valve is on the outlet of the water tank and should not be the cause of your problem… the inlet has no check valve and the winteizing valve is open so some heated water could be pushed back into the cold water line due to expansion and cause some mixing of the two…
although continuely alternating from warm to cold without changing the facuit is puzzeling to me… temparaley getting warm for short durations can be normal and come from several sources…

I would guess the facuit but now its on more than one facuit and kinda rules that out…

OPtions:
3-way water heater bypass. Bypass valve open
1 or 2 way valve(s) not fully seated in end of quadrant
Faucet washers or ball or cartridge worn out.

Fact: Nothing special about the faucet, a common DELTA or MOEN from your choice of plumbing store will replace it.

atreis wrote:
RV water heaters have a check valve that keeps hot water from flowing out their inlet, and this can fail. In that case, hot water can get drawn out of the inlet causing the temperature to alternate between hot and cold when just the cold is on.

Another possibility to check into - it's more likely to be the case if you have this experience with more than one faucet.

This might be the case.
My wife mentioned that the bathroom faucet also would run warm and cold.
And it is just as you state - it will alternate between hot and cold when just the cold is on.

So. . .I guess that I need to find the hot water heater inlet check valve?

RV water heaters have a check valve that keeps hot water from flowing out their inlet, and this can fail. In that case, hot water can get drawn out of the inlet causing the temperature to alternate between hot and cold when just the cold is on.

Another possibility to check into - it's more likely to be the case if you have this experience with more than one faucet.