You are correct about the single flash code, engine coolant temp too high. Sounds like you have check all the correct things first. It could be the thermostat, water pump, temp switch. With the water not getting warm and the short amount of time that the unit runs, I'm going to guess the temp sensor has failed. If the gen is on a nose slide and can slide out of the coach you do not need to pull the unit. You have to unbolt the top cover and remove. This will give you a large amount of access. If needed once the top is off you can then remove the side covers for more access. The coolant temp sensor is located on the top of the motor, on the left side of the block toward the front. There is also a water pump belt that can also break, Check that while your in the unit.
Most of the time if the temp sensor fails it will throw a higher level code to the computer of "27" for temp sender fault, The start button will be blink 3 times, then a pause, then 3 more blinks and a pause, this is a service needed fault, when you see the three blinks, press the stop button one short press and the second level codes will start to flash. Count the flashes, example, (27) two blinks, pause, 7 blinks. and the code will repeat for 5 minuets. Check the flashes again and let me know what you come up with.
After check the specs there are 2 passable problem. Here is a short cut from the manual.
FAULT CODE NO. 1
(Engine coolant or inverter heat sink temperature exceeded design limit)
Corrective Action: Check the second-level fault code by touching stop. The second-level fault will be eather No. 33 or No. 34.
CODE NO. 33 -- HIGHT ENGINE COOLANT TEMPERATURE FAULT
(Engine coolant temparture exceeded design limit)
1) Check the engine coolant level.
2) Check for and remove any objects blocking the air inlet or outlet openings in the bottom of the genset.
3) Reduce the number of appliances connected at the same time. (Note that high altitude and high ambient temparture decrease engine cooling capacity)
4) Look for a broken or worn coolant pump drive belt and replace as necessary.
5) Remove the air intake grille in the skid-base and check to see that the cooling blower is secure and that the blades are not fouled.
6) Check for air leaks in the housing
7) Clean and flush the cooling system to remove coolant passage fouling.
8) Remove the top housing panel, If loose reconnect engine harness spade terminal connector E3-1 to the coolant temperature sender.( If the lead is loose and touching ground, the controller will sense high engine temperature)
9) While the top housing panel is off, check for fouling of the radiator air passages, clear out and debris with compressed air directed from below.
10) Replace the coolant thermostat
CODE NO. 34 - HIGH INVERTER TEMPARTURE FALUT
(Inverter heat sink temparture exceeded design limit)
1) Check for and remove any objects blocking the air inlet or outlet opening in the bottom of the genset
2) Remove the air intake grill in the skid-base and clean the heat sink fins if they are fouled. Also check to see that the cooling blower is secure and that the blades are not fouled.
3) Reduce the number of appliances connected at the same time
4) Replace the controller assembly if the genset cannot carry rated load.
As far as testing the sensor you would have to pull the sensor out of the engine, place it in water with a meter connected to it on the ohm setting, put at temp gauge in the water and start heating the water. Note the water temp when the sensor trips. I believe the sensor should trip at about 265 degreese. Should have a temp reading wirtten on the side of it.