talltimo, it could be something as simple as a dirty electrode. First, a few questions -- when you say there is just two seconds worth of hot water, do you mean you turn on the faucet and then it stops? Or that the burner goes on for two seconds and stops?
Anyway, you almost certainly have a dual heater -- it works off electricity and off of propane. I know nothing about the electric side -- mine is just propane. But if you are not plugged into 120v power, I am pretty sure that the only thing that will work is your propane.
So let us simplify. Unplug from shore power. Make sure your battery is charged and connected. The propane won't work without some battery power to control the circuitry.
Open the hatch that covers your water heater so you can watch what is going on.
Now, go over to your control panel. Make sure that the water pump is on (unless you are connected to city water).
Turn on the water heater switch (or have someone do it for you). Go stand by your water heater.
Here is what should happen -- you should hear some clicking sounds (the igniter) and then the flame should ignite with a big whoosh.
I am betting that instead of staying on, the flame will soon go out. On your control panel, a little light labelled "DLI FLT" will come on. I think that means "direct spark ignition fault."
That was happening on my six year old trailer. I figured out that it was due to corrosion on the ignition probe. It sounds scary, but it is not, and it is an easy fix. Here is a link to a thread I posted elsewhere on how to remove that probe and clean it and re-install it:
My solution to the corroded probe
I hope that works for you!! Please let us know what happens.