Electrical failure

Jump in the van all normal! Key in, start! all normal,driving from engine and all mehcanicals 100% normal/perfect

So where the problem ????.. all systems running as mentioned, fine!.... all electrically 100% ok!

At any given time during the ongoing journey I will notice the fuel gauge drop to zero! (all driving 100% ok) .... as the gauge drops to zero, symutaniously I lose electric windows, horn, heater, wipers, all external lights without exception..... the cockpit lights all function normally at this time including indicator awareness lights... if I stop the van, turn the ignition off and back on everything re-energises and works again 100% ok until the next intermittent out time
Whilst I fully understand intermittent problems especially of the electrical nature are not easy... my local auto electrician charged me £25 and handed the van back not having any offer of rectification .....

So guys n gals anyone out there with the abilities to point me to a line of repair ??
Thanks Paul