I have a Compass 23TB also. I found trying to do anything through the little access hatch in the rear compartment is nearly impossible, (at least for a fellow my size.) It is much easier to simply remove the outside shower by removing the screws around it's perimeter, pull it out and you will have full access to the water pump and screen filter.
I found my screen filter to be packed full of what looked like white drill chips from when they drilled holes in the water tank.
If you find drill chips in your filter, I would also suggest you remove the water tank fill port from the side of your motorhome where you will probably find the tank vent hose is also plugged with chips.
In regard to the water heater, the only adjustment is a flow control which you can access by removing the drawer above the electrical panel. I don't recommend you change the flow control until you have exhausted other possible causes for your trouble.
The water heater will not work correctly if there is air anywhere in the water lines. Make sure you flush the toilet, run both hot and cold in the shower, kitchen sink, bathroom sink and don't forget the outside shower. Run water through all the fixtures until sputtering stops.
We have found the best way to use the water heater is to set the thermostat at a temperature you find comfortable, (I like about 102 degrees depending on the incoming water temperature) and turn on only the hot water in the shower. If the water heater is cycling on and off there is most likely still air in a line somewhere.