If the Fuel Display is working in Singleplayer, it should work in Multiplayer. So you get a blank fuel display when you start up your LFS?
I start lfsrelax, then I start lfs and type /insim 29999. Then I connect to a multiplayer server, everything works fine.
The only problem is that sometimes when there is a track change or something during a multiplayer session I need to close lfsrelax and restart it to get everything working again. I don't have to type /insim 29999 again, just restart lfsrelax. Then the fuel display starts working again.
Hope this helps your troubleshooting.