And your 2 year warranty just expired, right? My dealer replaced my outside shower under warranty, because it dribbled.
But, if the above doesn't apply:
I'd blame the valve inside the "Hot" knob. These are all very low end plumbing fixtures. It is pretty easy to take the knob off (hint - pry off the "H", to get to the screw underneath to remove the knob). Then you should be able to figure out how to unscrew the valve assembly.
Leave your heater off - you don't want to play with hot water.
Unscrew the hose and shower head from the fixture.
Have someone turn on the main water (stand back!) If you get a gusher out of the valve hole, you have good supply to the valve. If not, time to look at the internal piping from the heater to the valve.
While it is gushing, try plugging the valve hole with your thumb and or a rag (yes, you probably will get a bit wet). If you get a decent flow out of the "faucet" that the hose screws on to, you have no blockage between the valve and to the outlet. If not, time for replacing the entire fixture (or mess with it to see if you can unblock the passage from the valve to the "faucet".)
Inspect the valve for crud that would block the smallish holes in it. If there was anything (scale build up, etc.) , put it back, and see if this resolves the issue. You could also remove the Cold one, so you can compare a "good" one to the "bad" one. A little study will probably reveal whether there was crud that can be removed, or if you have a defective valve that doesn't open much.
If you really want to be sure it is the valve, swap the hot and cold ones, and see if you now have the problem on the cold side.
If you decide the valve is bad, take it and the knob to Home Depot (or similar). Have them find a replacement that matches yours. Note that many times, the available replacements fit perfectly, but the way they accept knobs may not allow you to reuse yours! Hence why you have yours with, to verify without having to make another trip. If it won't fit, you can buy replacement knobs there that do fit. In that case, buy two valves and two knobs and plan on replacing the cold one too, unless you don't care about your knobs being "matchy-matchy".
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder