This is the classic symptom of low voltage to the heater under load, so please check the voltage at the heater power input connection with a multi-meter when you initially switch your heater on. See point numbers 1 to 4.
1. Check the battery battery voltage going to your heater. It should be: 10.5V to 14V (12V system) or 21V to 30V (24V system) under load.
Many heaters have failed to operate because the battery voltage has become too low due to lack of battery charging or excessive power drain from other ancillary equipment or faulty batteries.
2. Check the wiring harness or connections for corrosion, the build up of corrosion around a terminal or battery connection will hamper the connection or the batteries ability to allow power to pass. Whilst checking the condition of a terminal, don't forget to check the terminal or connections security.
A loose terminal or connection can have the same affect as corrosion.
Maybe cleaning and applying some corrosion inhibitor (e.g. petroleum jelly) will prevent this problem from occurring again in the future. A routine check of all terminals and connections is always a good method of preventative maintenance with any vessel or vehicle.
3. Check the condition and rating of the fuses used on the heater ensure they are in good order and the correct amperage. As stated in 1.2 above, corrosion and loose connections can cause problems.
4. Undertake a visual inspection of the heater and its associated parts. Is there any physical damage to any component parts as this may have a detrimental effect on the heater's ability to function correctly?