I have never owned a 535 but this does not sound normal revs for the speed you quote.
My 94 e34 525tds auto would do 1800rpm at 80mph ish.
I am going to presume your car has an auto box the same as my tds as in ecu controlled 5 speed with 5th being done courtesy of the lock up.
The 'load sensing' you refer to is done by the ecu's, both the gearbox and the engine management communicate with each other and plot gear changes dependent upon throttle position and road speed.
Now it is a bit more complicated than that but that is the overall conditions it operates on.
Now for instance my tds would lock into 5th gear just over 30/35 mph on a nice slow cruise,but if there was a very strong headwind then it would lock up at a higher speed and sometimes not hold 5th for very long before changing down to 4th.
This is simply because the head wind required more throttle pedal input and hence the criteria to change to 5th gear would not be met.
Cruising in 5th and approaching an incline and starting up it would see the box change to 4th to hold the road speed and then once over the top it would go back into 5th,all this is without using a cruise control or altering throttle pedal position.
The box on mine was a 'learning box' all this means is that it learns your habits and uses a few inbuilt programs to suit your driving style.
If you drive hard then it will always start off in 1st gear.
If you drive soft then it will be 2nd.
It is possible to confuse it a little and drive hard for around five minutes and then drive soft,you will find it holds on to revs a while through the gears and then after a while it gets the idea and changes up early.
Now once it is changing up early you drive hard and again it is not sure what you want it to do and it feels hesitant and will let the revs build before a change but feels like it is not letting you have all the engines performance,after a few minutes it then goes back to race track mode.
So between these two shifting scenario's you may find the odd slurred change or odd behavior so do not think the box is going bad it is just confused.
Disconnecting the cars battery resets all that it has learned and things can start afresh ,most dealers do this when the car is serviced because when you drive off you notice the difference and feel what money you have just passed over has made a nice difference.
Anyway back to yours,you are expecting a fuel filter or fuel issue,if the engine struggled for fuel then yes you would require more throttle pedal input to hold a certain speed than you would normally and in this instance this would delay the change to 5th or stop it all together,binding brakes would do exactly the same.
Not sure about the vacuum leak ?,mine did not have a vacuum connection,the box was a sealed for life with no dipstick.
If you have no speedo function then this will mean your diff mounted speed sensor is out and not giving a signal and this will mean no 5th.
You have no trans warning so all is good,this will normally surface due to low system voltage or an engine sensor going down and the ecu shuts the box down to limp mode to protect the engine/car.
Trans program rarely means a bad box,in fact the only thing that the ecu can detect wrong with the box is slippage,and this is picked up from the speed sensors in the gearbox itself,you have no light warning so your problem is most likely with the fuel system as you have suggested.