Hi and welcome!!!
First, the RV thermostat works on 12V, it should only affect the air conditioner or the furnace. No power to the slides, and not able to start the generator initially (also 12V) suggests a battery, most likely the house battery, is bad and/or is not being charged for one reason or another. By starting the engine (chassis battery) it may have supplied the house system with enough 12V to start the generator.
Check the house battery by checking the water level in the battery first, adding distilled water if necessary. Then check the voltage right at the battery terminals while the coach is plugged in and again a little later on when it is no longer plugged in. While plugged in the voltage should read at least 13.5V or so, not plugged in.... you need to tell us. If, when plugged in, you do not get 13.5V or so, you have a converter problem and need to figure out why that is not working. Could be as simple as the protection fuse(s) blown, or no 120 V to the converter. Don't forget to check the GFCI outlet as well is it could be the culprit in not supplying 120V to the converter in some cases. (Makes and models would be nice to have if the converter appears to be the problem.)
