If the fire is coming on and staying on as long as the thermostat is calling for heat, I'm short on any ideas of why it would not be warm enough.
There are a couple points that I know can mess it up even with the fire burning right but they are kind of odd things.
If there is something in the path where air is pulled in through the section that gets hot and enough to slow or stop the air flow, it usually shows up as not burning or going out, so I would not guess that.
But the fu4rnace on RV are kind of like a blowtorch in a barrel with the inside air just passing over and around the barrel, so there isn't too much really hard stuff if the fire is burning right.
There is a limiter switch that can go bad and make the fire go out before it should. It's meant as a safety thing so that if the furnace metal and around it gets so hot it might cause a fire, it shuts off the furnace. But that doesn't sound right when you say it is not warm enough. The fire would go out and just stop! Not warm, just dead!
I got a picture of where the warm ducts run, just for ideas but if they seem to all be getting near the same amount of air, that may not be any help.
The longer may be a bit cooler if the air has to travel further but not after it gets them all warmed as it runs. You've got one that is quite a bit longer and might take a minute or so longer to get it warmed up?
I was thinking one might have fallen off the duct connection but flex duct will not have very many connections along the way.
If the fire is coming on and staying on as long as the thermostat is calling for heat, I don't think of any real way to change the amount of heat.
No adjustments I think of but just fire on until the thermostat tells it to go off!
If it was coming on and going off too soon and then starting again, it might be things on the furnace controls or thermostat??