Driving alongfreeway abiut 100 klms hour all going fine 1 moment then out of the blue there was nothing, no gears, just reving like in netural. no noise was heard either

best at 10 liters per 100 klms , average around 12 liters /100 klms
fuel consumption depends largely on load , speed , operating conditions , towing, highway/city, off road so an exacting answer is not possible as it all depends on the operator and the driving skills

the towing procedures will, be in the user manual
depending on the trailer weight( less than 1 ton) it is permissible to tow in over drive and for a trailer without load levelling ( stabilizer bars ) 100 klms would be an ideal speed
5th gear is a direct drive gear ( input and output shaft connected as one) and the idea of staying in 5th gear reduces the load on the gears and bearings

The only thing that comes to mind is that in 5th, the rpms may be so low that you are running on low speed potion of ignition module, getting weak spark and not burning fuel properly. This could be true in all gears at low rpms, but you would not notice it as much because you probably keep the revs up more in lower gears. Just a thought.

Sounds as if the spider gears are going bad. There are bands or belts in your tranny for each gear. They may be going bad. Have you checked the transmission fluid level? This also could be the most obvious reason. Just
Low fluid level? Noticed any drips in the driveway ?

time for a trans rebuild.
Best bet is to get it to a shop as it just may be something easy, but as a chevy mechanic, I honestly HATE every GM transmission.

You have a failed transmission, have it removed and replaced or rebuilt.

Should be under warranty from the $2800 repair...take it back

First check to see if the driveshafts are turning, they are the things that stick out either side of the gearbox. start engine put car in gear. and let the clutch up, if you have no drive, open the bonnet and see if the gearbox end of the driveshaft or the wheel end of the driveshaft is moving. if one or the other is (remember to look both sides of the gearbox 2 driveshafts) you have a driveshaft or hub fault. If all is well and nothing is loose or moving the problem is either clutch or internal to the gearbox. this will require removale of the gearbox. approx 5 hours labour remove and refit plus parts required on inspection.

i'm not on my other computer now , but i hope and you see this message, and i have not set this computer for the email service,
ok i will recomend you to change the oil and filter, seems to be like the filter inside the transmission fall of internally, or might be the o-ring holding the filter is worn out, do that and let me know,
i will also apreciate your feedback, since it is the only thing thet keep me here answering questions,
and if that does not take care of it, then let me know or haVE
some one to check for any transmission codes present, this will be done with a diagnostic machine. thank you.
and excuse the delay but just been so bussy with my customers

i may be the speed sensor