Hello HNB,
I have tried to find a picture or diagram of your layout without success, so I can't off detailed advice, but there are a number of points I would like to bring to your attention.
The most important question is whether such an extension will actually work. Hot air systems area good solution for shorter caravans, but as the duct length grows, it compromises the ability of the fan to blow the hot to its furthers point, so even if you can extend to that distance will it produce any warm air at the furthest out let?
The principle factor here is the pipe exerts drag on the blown air slowing it down, The long the pipe the more drag and the greater the loss of velocity. Its one thing of the pipe is straight, but for every 90 degree bend in the pipe, its the equivalent of adding and about an extra meter to the straight pipe length. Obviously any other air outlets along its length will also rob flow to the extreme end.
The slower the air is moving through the pipe the greater the opportunity for it to loose its heat through the duct walls.
Unfortunately is no simple formula to predict the effectiveness of an outlet of the type you are proposing, but there are some generalised ideas.
Keep the ducting as straight as possible and avoid sharp bends.
If possible keep the ducting inside rather than out to reduce waste heat losses out side. It may worth thinking about running a duct along the bottom of an inside wall and arranging to box it it to protect it - some manufacturers used to do this on side dinettes.
If it has to go out side, it is essential to ensure the duct is contained in a waterproof jacket (Truma used to sell a waterproof outer duct) and it is obviously a good idea to insulate it. There is a dedicated elbow for going through the floor, but its not essential.
https://dealernew.truma.com/_anweisungen/Truma_Katalog/pdf_verzeichnis/40_000/40000_35300.pdf
IF air flow is weak, Truma sell a "TBM Multivent Fan" See item 15
https://www.truma.com/int/en/heating/accessories-s-heaters.php
https://www.truma.com/downloadcenter/multiventtbm_installation_instruction_de_gb_fr_s.pdf
which can be added to a length of duct to assist air flow.