different dc readings

I'm not familiar with the specific dimmer you show, but dimmers of that ilk that I'm aware of generate pulses of DC of varying width (i.e., PWM or pulse-width-modulation) to adjust the effective output voltage.  If you have, say, 12V DC going in, the dimmer knob adjusts the output pulses from very narrow to very wide. The pulses occur thousands of times per second.  At mid-scale you would get 12V DC pulses that are present 1/2 the time so the average voltage is 6V DC as measured by a DC meter...but to be absolutely clear, this 6V effective voltage is a stream of 12V pulses.  More on this later.

It is common practice to put a small capacitor on the output of the dimmer which will charge up to the peak voltage (~12V if you have 12V going in).  If there is no load, the capacitor does not discharge between pulses and it stays at or near the peak voltage.  With the load (i.e., with your LED strips hooked up), the capacitor does discharge between each pulse so you get an average voltage which should vary from 0 to the DC input voltage.

But, in my opinion, you should wait for the AC-to-DC converter to test the Menards building which presumably wants 4.5V DC or less if dimmed.  The AC-to-DC modules that are discussed on OGR will have smooth DC (not pulsed) and will maintain their regulated voltage with or without load.

That is, even if the average (as measured by a DC meter) is tuned down to 4.5V DC or whatever that voltage is still a stream of 12V DC pulses (if you have 12V DC input).  This is OK if driving a nominal 12V DC LED strip as it appears you have since those strips are meant to be driven at 12V DC (or less to dim).  BUT other lighting circuits may not be designed to handle the 12V pulses even if the average voltage is 4.5V DC.  Some may chime in and say it should/will work with some Menards buildings that only have LEDs and so be it.  But I'm pretty sure there are Menards buildings that have more complex electronics such as to drive those "neon" signs or spinning rooftop fans, and who knows what else.  It's not clear to me those buildings can tolerate the 12V pulses. 

Bottom line, I don't recommend getting into the mindset of using these DC dimmers as a proxy for an adjustable DC voltage regulator for testing or whatever - especially when there are countless low-cost voltage regulator modules or even DC bench supplies for just a few bucks.

Stan, i just came up from the layout & i decided i would put a small length of strip led's hooked up to the dimmer to see if putting  a load on it would make any difference & i was able to adjust the output voltage much lower than with no load ,i think it was somewhere around 2.6 v ,or close to that ,however as you have stated in your reply i don't think i should try & hook up the strip mall to this dimmer i might burn up all the led's ,it was just a thought i had as i have not tested the lights on the strip mall since i 've had it ,but i will wait until i get the regulator  ,it may be awhile though as the tracking number indicated that shipments from China may be delayed ,but thanks for a very detailed response .