3000rv cargo heater doesn't work

SOURCE: 1996 buick roadmaster keeps overheating:

A shotgun approach to repairs can really rack up the bills.

If your symptoms are loss of water and you have replaced the thermostat, you more than likely have an issue with steam. And that steam more than often comes from a bad head gasket. Most head gasket gasket failures seem to be confined to the water side initially, but let it go on bigger damage will occur.
You can buy a litmus test strip to dunk in the coolant. It will indicate if there are exhaust residuals in the water and thus a gasket failure.

However it may just be something simple, given that you have just about replaced everything:

have you given the radiator a good hosing inside and out?
is the radiator fan running when it should be?

SOURCE: i have a 1997 dodge

Only way to get smoke (steam) billowing in is a faulty heater core. It won't steam until the engine is warm. You should also be smelling an odor unless you have pure water in your cooling system. That vehicle requires the removal of the steering column and entire dashboard to replace the heater core.

SOURCE: heater fan speed control only works on 3,4 & 5

No, it is not a fuse problem. Most likely you need to replace the heater blower resistor. A replacement should be available at your local parts store, and they should be able to help you find it's location. If you follow the wires from your blower motor, they will lead to this component. Usually two 7mm screws hold it in the heater box, but might be 5.5mm (7/32").

SOURCE: 2010 Elantra Touring - car won't start -

don't forget to check starter, and ignition switch,otherwise i think John Parker is
right on the money.
I also think that if the shop couldn't find it they shouldn't charge you at all.

SOURCE: My 1996 Olds LSS doesn't idle properly,idling

Your first problem is to find out why the intermittent no-power problem. Honestly, that sounds like a loose conmnection somewhere. check all your grounds for clean and tight conditions Also, battery connections and alternator connections. Double check your power distribution center for any loose wires or poor connections. Once you solve that problem, then you can diagnose the other problems with poor idle and temp. gauge.