Charging battery on the road with portable generator

50 years ago before all the modern stuff we used a continious duty solenoid and a switch in our TC to charge the extra battery that powered the TC. 1 battery? We never had all the extra electronics. It was ice in the ice box, no furnace, and the TCs had an oil lamp mounted on the wall. Anyone remember those old oil lamps mounted on the interior wall of the TCs? We were just excited about being in a hard sided unit up off the cold hard ground. Now we take our house with us. How times have changed.