Low Fuel Pressure

Your mechanic is correct!

Any air in the line to the sensor will be compressed to the exact same value as the pressure in the line.

The sensor measures pressure and it does not care and can't even tell if the pressure is coming from a liquid or a vapor.

Pressure is pressure!

I have a 2010 ELSA RV12 that I am having fuel pressure problems (I think) with.  I never paid much attention to the fuel pressure until recently when I got a low fuel pressure warning on climb out.  Pressure was around 2psi.  Throttled down and landed and the pressure came back up.  Flew back to my home AP with no problem until I throttled ed back to land.  Pressure dropped to.9 but returned to about 4 after landing. 
I ran the engine up to temp today and pulled the fuse for the electric pump.  Pressure dropped from about 4.1 to about 2.1.

What should pressure be with just mechanical pump and with just electric pump?

The pressure should be 3-5psi with either pump singly or both together.

The electric pump takes the load off of the mechanical pump but should not affect the pressure significantly.

You might see a 0.5 to 1.0 psi difference, but the tow pumps do NOT add together.

The 4 to 2 psi change might be reasonable if the engine was idling.

Does the pressure vary with power when using the mechanical pump only?

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You are seeing pressure drops at times of high fuel flow in the climb.

The Fuel filters would be the first suspects.

The drop when throttling back is commonly caused by high heat under the cowl in conjunction with slow fuel flow in the lines associated with the low power setting causing the fuel to "Boil" A.K.A. "Vapor lock".

The electric pump pushes the vapors into the carb and the problem clears itself as the temperatures drop.

Thank you for the feedback Bill.  I assumed the pressure should be the same with either or both pumps.  I don’t believe the pressure drop on throttle back was vapor lock.  Temps where quite mild and engine temps were not high on EGT, CHT or oil after about a 30 min flight.
 I have experienced vapor lock this past summer with very high ambient temperatures using Mogas with ethanol.  I have been using 100LL since with no problems.  Annual Condition Inspection was done in November and gascolator fuel filter was inspected and cleaned.
I purchased the RV in March and received my Sport Pilot certificate in September so I am a novice.  

The low fuel flow indications you are observing are an oft-reported issue on the RV-12 / ULS.  Van's has published a Service Notification to hopefully address this issue.   

https://www.vansaircraft.com/service-information-and-revisions/n-2018-07-12/