Hi there. I have flat towed several MGB's behind our motorhome with a towbar and no problems. I did not disconnect the drive shaft as I wanted it to be able to drive. Towed 1300 miles and more over few days. To hook up the tow bar brackets I removed the front bumper over-riders only and drilled out the mounting hole to 1/2 inch to accept the bolt for my swivel brackets. Then using the grade 8 bolts I mounted the brackets to the front bumper face bar through the bumper mounting brackets. Leaving the bumper on braces the front frame brackets. Then I made a simple connector to the wiring plug on the motorhome with a mating plug-in and about five feet of wire. Strip and solder the wire ends to about 1/2 inch and then you can slide the soldered ends into an appropriate bullet connector between the rubber sleeve and the contact tube to give signal, tail and brake lamps. Leave the harness coiled up under the bonnet and you are ready to tow again. I had no problems at all and felt the whole hookup was strong enough for highway towing. Don't try reversing though.
Cheers and good luck. Dave