Hello Karen,
All Carver heaters that used the Carver installation box could have some form of blown air heating fitted.
It was an optional addition, which became increasingly popular towards the 1990's and beyond, when it became almost standard equipment in most caravans.
Blown air systems could be retro fitted on most heaters where the Carver installation box had been used. The box had a pre-perforated location for the fan mounting.
Some makes of caravans (most notably ABI) did not use the Carver installation box on the SB1800 models and so blown air in these was not possible.
There was a version of the Caravelle heater that had front mounted fan. This system could not be added to any of the other models.
The 1800 size of heaters was only suitable for small fan systems (Minor/Junior systems) Where as, the much larger 3000 and 5000 series needed the full size fan kits.
From the early 1990's, the Fanmaster system was introduced that incorporated electrically heated elements. This allowed the caravan to be heated by gas or electricity where the supply allowed it. Again these systems could be retrofitted.
Towards the end of the 1990's, a new range of carver heater were introduced that had the electric heating and fan systems built into the basic heater. These later systems could not be added later unless the whole heater was changed.