This is what Truma advise:

Short description
Heating is basically possible in gas mode, electric mode and
combined gas and electric mode.

If the heater is operating with gas and electricity at the same
time, electric mode is switched off before any possible overheating
can occur because of the more powerful gas burner.

In Electric mode we recommend operating the TEB-3 or
TN-3 fan at level 3 (manual or automatic) based on an operating
level of 2000 W (be sure to comply with the power
supply fuse protection at the campsite).

If heating power of more than 2000 W is required (heating up /
cold outside temperatures), the heater must be operated
in gas mode, since 230 V electric mode is only secondary
heating.

Heating is also generally possible without the TEB-3 or TN-3
fan with the S 3004 / S 3004 P heaters independently of the
operating mode (gas or electric).

However, in order to ensure that the warm air is distributed
evenly and quickly and to lower the surface temperature of
the heater unit, we recommend leaving the fan and a warm
air system on at all times during heater operation.

If your Ultraheat additional heater is being operated with a
remote sensor (accessory) it must not be placed in the vicinity
of external heat sources (cookers, lamps, opposite warm air
outlets etc.) or in locations in which heat can accumulate
(below shelves, in corners, behind curtains).

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Although the manufacturers say that the electric element is only designed as supplementary to the gas unit, we have never yet found it necessary to light the gas fire other than to test that it would work should the electricity supply fail.

Quote: Aaron Calder wrote in post #52

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Heating is also generally possible without the TEB-3 or TN-3
fan with the S 3004 / S 3004 P heaters independently of the
operating mode (gas or electric).

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