You're sorta kinda in a bit of trouble here. This system is close to being good, but because of a seemingly minor screw-up, you're going to have major problems here. The recirculating pump SHOULD NOT be connected to the drain.
First off, it's quite possible that the segment of pipe between the pump and the "tee" attached to the drain valve is completely or partially clogged with (corrosive) scale/sediment buildup. Judging by the scale buildup on the outside of the pipe just below the tee, it looks like the tee is leaking (why? corrosion?). I can only imagine that the inside is worse. Even the check valve and the ball valve upstream from the pump look like crap. In fact, I'm pretty sure that's calcium on the ground below the check valve (thing between the pump and the drain with a hex bolt -shaped top on it), indicating that it too is already leaking. The check valve almost certainly isn't functioning properly (check valves are intended to only allow water to flow one way,) which is possibly why you are having to constantly readjust the temperature in your shower. The cold water gets injected into the tank at the bottom (via a long "dip tube" that enters at the top and travels down to the bottom.) If this check valve is being held open by sediment, or if it has failed due to corrosion, then cold water could be coming out of the water heater from the drain at the bottom, making your "hot" water turn merely "warm" after about 10-15 gallons of use, which is one possible explanation for your "shower get[ting] colder during use" problem.
My advice would be to get this junk removed ASAP before something bursts and you have a disaster on your hands. Get rid of it completely, or if you want to get it fixed up, get a new pump (Grundfos makes great pumps, but I can't imagine that this pump is usable at this point) and tell the plumber to connect the recirculating pump to the T&P relief valve inlet, which is not currently being used(?), rather than the drain. Doing this would reduce the scale/sediment buildup greatly, which happens mostly near the bottom of the tank. Replace all of the pipe in the vicinity of the pump as well, as it is probably completely scaled up and corroded. Everything else should be ok.
I see a line (the CPVC that goes into the wall) for what I hope to be a T&P valve, but I can't see where the actual T&P valve is. If it's tee'd off of the hot water outlet, then that's fine. I'd hope it's not simply laying on top, not connected to anything, but I don't have enough faith in whoever installed this to take that for granted.
I agree, your symptoms seems to indicate the problem is the recirculation system, not the heaters.
First, locate the recirculating pump. It could be near the water heaters, or at the farthest fixture from the water heaters. Your most distant bathroom may be a good place to look first. Look under the vanity or other enclosed places for a smallish device (size of a closed fist or bigger) with two water lines and an electric line.
It may have a timer that is mis-set, it may be unplugged, or plugged into a dead outlet (perhaps a tripped GFCI).
Other possibilities would be a valve closed either near the recirculator or at the water heaters (if the recirculator has a dedicated return the whole way back).
If none of these lead you to the problem, you may have a defective pump, or a more subtle problem... it may be time to call a plumber.
Best Answer
Some recirculation systems are based on gravity/thermal expansion effects that create a small pressure differential to keep hot water moving and recirculating through the pipes, from the hot water output back to the recirculation input.
This is a feature for comfort, and it comes at the expense of efficiency: hot water continually leaves the heater (albeit only a very low flow) and returns a bit less hot after having been cooled by the piping, all this while never having been used for washing etc...
This same gravity/thermal expansion effect that is exploited for recirculation can also cause an undesired flow of hot water out of the heater. Hot water leaves the heater and returns to it a bit less hot, all in the same outbound pipe. It's convection within a pipe, much like convection in a tall and narrow upright cylinder.
This undesired convection can be stopped by installing a P-trap or a nipple. Both create counter pressure to stop the flow: the P-trap by the gravitation and counter convection by the P shape of a section of pipe, and the nipple (a short piece of pipe with two male threaded ends) by the elasticity of a membrane or a ball inside it. It will take more pressure than that provided by convection to cause water to flow through them.
Two traps are shown here, one is based on a heavier than water ball, the other on a membrane.
When a faucet is opened, the much larger pressure from the water mains will easily push through the minor counter pressure, allowing water to flow as expected, but only when a fixture is open.
The counter pressure to prevent convection losses can form a problem for recirculation pumps that are designed for low flow at low differential pressures. Thus, a recirc pump that intends to pump water at a very low rate through a heater with nipples in the lines will need to be slightly more powerful than one rated for lines without such counter measures.
So if a pump is not rated for heat traps, it means it cannot overcome the heat trap's counter pressure. If the heater or its installation enable recirculation based on the same gravity principle, then the pump most likely can push through it, as it will aid the gravity induced flow, not counter it.