1) No disconnect needed. Although I recommend plugging the portable into the generator first to get it started.
2) Generally the battery switch needs to be on to get a charge from the on board converter. Most modern converters will power the RV fine without a battery. Lead-acid batteries prefer to be 100% charged and held at 100% charge for longer life.
Not enough battery and not enough wire for 1500 watts.
If the heater has any electronic controls it may not like the modified sine wave (MSW). Incandescent bulb does not care about MSW.
Heavy jump cables would be 1 awg like these:
You really should have 400+ amp hours to drive that inverter.
If you plan to use it to watch tv fine but to run a heater I think your battery will be dead before you feel much heat.
Battery Tender is an excellent brand. However 0.75 amp is a little light for 200+ amp hours of battery. For a few dollars more I recommend the waterproof 5 amp version.
Battery Tender 022-0157-1 Waterproof 12 Volt Power Tender Plus
OK maybe it is more than a few $$$ :(
You should also have 2 to 4 batteries and LED lighting for camping off-grid in the RV.
1000w will drive about 40 amps into the battery. If your converter is larger than 40 amps it is probably best to get 2000 watts. The 2000 will also run the microwave in most cases.
I always recommend 100w to 300w solar as a compliment to the generator or possibly instead of a generator.
That is maintenance only for two batteries. I recommend fully charging and disconnecting a cable for best results in storage.
For $125 you could get a 100 watt panel then a low cost controller for a system that will actually do some charging.