Welcome to Sunline Club. Now let me see if I can help any. You have 2 questions.
The LP tanks. A number of years ago they changed the style of fill valve to have an inboard float to only allow 80% tank fill. That is the new valve I believe you are referring to. Called and OPD valve, (Overfill Protection device) The new style looks like this. And you can tell by the valve knob. The new style has a tri shape 3 sided knob. The older ones have a round knob.
By law, propane dealers can no longer refill the older tanks with the old valve if the tank is more then 12 years old and not recertifed. And most now have outdated.
And now they have a 12 year inspection law on the DOT type cylinders. This came along at the same time as the OPD valve. Stamped on the collar, (ring around the valve) of your tank is a mfg date. If the tank is older then 12 years it has to be inspected. And then after that it is every 5 years.
This link helps explain this, http://www.propane101.com/propanecylinders.htm
Now comes the question, well where do I get it inspected and recertified? And can they change the valve for me in the process?
That is a good question as the 12 year time frame has not yet come due on the original law, “I think” so it is not as common yet. I do not remember the exact year the law went into effect but it was not that long ago and none of my tanks have come due yet. The place I go to get my tanks filled have lots’ of old non OPD valve tanks just sitting there as a collection spot. In fact I had to turn in 2 of them. And it may be that the price to recertify and add the new valve my not be cost justified as compared to buying a new tank.
So if you can find someone to recertify the tank and replace the valve and cheaper then a new one, it might pay. I would start with a local propane dealer. Not the BBQ grill refill places.
Now on the furnace. We sort of need a make and model number to help dig deeper but here are some big picture things.
If the blower runs but no ignition, then this points to at least the Thermostat is working and so is some of the furnace. You said it had direct ignition. I assume you mean Electric ignition. I did not think they had electric ignition back in 1987 on camper furnaces, but in case you do, do you here it going click, click, click when after a number of seconds after the blower starts?
The normal starting sequence is, the blower runs for a time length to purge the system. Then if all the safeties are met, it will open the gas and start the ignition. If no ignition within a short time period, it cuts off the gas and stops the ignition. It then Purges some more and try’s again. It does this 3 times and then totally stops the firing sequence and goes into fault. This is how most gas systems work. Without knowing more on your brand/model can tell much more.
But on the normal RV furnace there are the safeties.
A sail switch. This works like the name says. When the blower is running the wind pressure is large enough to move the switch and make the safety circuit. A weak battery will not have enough power to make the switch move.
An over temp switch. If the inside of the furnace gets to hot, this opens up and shuts the system down
Ignition or lack of gas failure sensing. If no flame occurs in x seconds, the system shuts down. This could be an ignition problem or a gas valve problem. Key is no flame in xx seconds it shuts down the gas/firing.
Now some furnaces have an on/off switch in line with the gas valve that is outside at the furnace. If that is off, it will not fire.
Has the furnace ever worked and now it is not?
Also it takes time to purge the gas lines when you just turn on the LP tank. If the range top is working, then you have a fighting chance the furnace will fire off within the first 3 tries when you turn the gas on. If no range top working, the there can be too much air left in the lines to work the HW heater or the furnace or fridge. .
Hopefully some of all this will help you tell us more and maybe we can point you where to go looking. It might be something simple or more complex
Hope this helps