I have a sealed in area I need a durable small room space heater. I have tried several brands they all seem to burn out or shut off after the room gets warm. Some were a waste of money it seems. None lasted very long. I want to keep the area 80-100 degrees for a sustained time period. Weeks - months.

These units most all likely have built in thermostats that are shutting the units down after it gets warm.I need something that is just going to keep running and not shut down. Or at least maintain the desired temperature I need without shutting down unexpectedly.

Any suggestions would be appreciated.

is this for your wine ?
water bed heater in a sub shelf below your brewing baby's

I'm assuming the thermostat only goes up to about 70º and that's the issue with not getting the space warm enough? I don't know how much area you need to heat but in storage rooms with stuff that can't freeze, we use Qmark brand wall mounted electric heaters. They're not cheap but they'll last a lifetime and you can set them up with a wall mounted thermostat set at any temperature you want to maintain.

https://www.gizmoplans.com/product/cozy-space-heater-plans/
Cozy Space Heater Plans
Take a look at this. Uses thermo-siphon from a water heater.

I used to tent camp in a wall tent for 2 weeks during archery season in W VA Some years the temps would be in the teens overnight and other times it would be in the fifties . We had a ventless propane heater with a thermostat to heat the tent It was set up to use a 20 lb to 30 lb propane cylinder as it's fuel source .The thermostat was set up so the heater would go off and on as needed to keep the tent warm But it did not shut the heater off .It would maintain whatever temp you had it set at . Even on the coldest mornings the heater had no problem keeping the tent at what ever temp you wanted it to be ..Another feature I liked about it was if you turned it off it would heat the tent up in a very short time ..There was no fumes and you could set it on the floor or mount it on a wall if yopu where using it it a building the had wooden walls .Depending on the temps a 30 lb bottle would last about 3 to 4 days .But depending on what temp setting you used and how air tight the building wasI am sure you could have the propane cylinder last longer. It was called a vent free gas heater and it was 20000 BTU

I was thinking propane also, propane is a damp heat, but if that isn't a problem I would go propane wall heater (either ventless or vented, your preference and connect it to a couple hundred gallon tank you can have propane delivered to. An electric fan option will heat more efficiently, but I would make sure and get a heater that will run without electricity (there are quite a that won't) so you have no worries if the power goes out.

The space is 8'x 8'x 8 'x' 20 Ft.

Our wall tent was made to sleep 8 .It was plenty roomy when we had a table in the center and 4 cots for 4 people to sleep inside Probably about the size you are thinking about .Perhaps one negative was the heat was more at the top than near the floor I agree that a fan would move the air a little more . And in a situation like you are thinking of a couple large tanks would be the easier way to set up

If they are not electric it could be a low oxygen sensor shutting the heater down. Try putting the heater up higher.

I assume you’ve tried a milk house heater?

This was my first thought or if too small of tank they can freeze from drawing too fast from the tank.