Wow thats a helluva set up u got there. Nothing like that around here thats for sure. from the looks of all those valves u may just have the 1 heater to do the water for plumbing as well as the baseboards. Thats not a good set up if thats how it works. But anyway 1 of those ( or more ) yellow handle valves should controll the water to the baseboards. Find it and shut it off when its not needed, but you better ck with ur plumber 1st cus i guess its possible they need some amount of water flow or pressure to prevent corrosion , etc...
The theory your working with is you have a appox, 60 gal tank there from the looks of it...When u have 60 gals of HW and you turn something on and use say 10 gals, then the 10 gals is replaced by room temp water...so the other 50 is no longer as hot as the 1st 10 was and the more you use the lower the temp drops as cooler water is added to the tank.
Turning up the temp will not give you longer lasting water but will make the decline in temp a lil slower. By the way it takes 140 degree water to sanitize dishes , so anybody with a DW should have their htg set to at least 140. I suspect there is something running constantly, like your baseboards maybe that is depleting the tank and never letting the tank reach the 12o u have it set at. That shouldnt be happening..
You may have replaced all that when your original problem is a bad regulator for the baseboards or something like that....
When your dealing with electricians, plumbers, a/c people ,etc there are 2 different kinds. 1 kind who diagnose and repair the problem and the other who just change everything cus thats all they know how to do...
In this case you need the service man since you've already tried the installer route...You Gas bill should be high as hell as well if the problem i described is whats wrong...