Temperature compensate both chargers niner...manually
At 70F The Mega would be set to 14.8
The Cheapo would be set at 13.4 MUST watch voltage to not over-watt the Cheapo.
Get Intermatic 6-Hour spring wound timer on eBay.
Without using a pair of rectifiers to isolate when Mega comes online the Cheapo goes to zero amperes. When Mega goes off line Cheapo automatically starts up. Limit hotel load to <25 amps and the Cheapo will do just fine. If you're going to take a hike dial the Mega back .1 volt higher than the Cheapo setting. No way can the Cheapo be overloaded that way.
Use micrometer dials on the 10-turn pots. Then make a chart. Voltage=Temp=dial number setting. 127 degree day on it's way? Set dial to 16. Pianotuna move in next door? Set dial at 882
They can take Plug N Pay and Stuff It.