solved: After some "sleeve" research I am now sure that the sleeve has to be installed following the workshop manual. I is a little bit tricky to get it to the right place it but in the end all seems to work fine. all parts fit together perfect, and also it makes sense to me to have it installed. I assume that a "mechanic" decided not to install it during last cleaning procedure to get it done faster ...
after disassembling / reassembling the heat exchanger three times I got some expertise with that :-)
@Bill Rouse - it is for the Volvo and not for the Onan. I attached a picture that shows the sleeve on the tube.
thx for your responses,