I have a propane furnace in a mobile home/trailer the air coming out of the vents is around 90 is this the correct temp or should it be warmer

90 sounds a bit high. I have an electric furnace and I know that the heat is not that high coming out of the vents. You must be going through a lot of propane each season. You should have the furnace properly calibrated.

This high a temp coming out of your vents could possibly pose a fire hazard. Do have it professionally checked as soon as possible.

Now this depends on the air temperature coming in AND the temperature rise rating of the furnace. Propane has a higher heating value that natural gas. If the furnace has a nat gas orifices in, then I'd be concerned.

Based on 70 degree entering air a 90 degree discharge temperature is just fine and could be a little low. That is only a 20 degree split. The downflow furnace in my mobile home at my camp actually discharges at 105 degrees at register with 68 degree entering temperature and it does just fine. Remember our temperature is around 98.6 degrees so anything less than that we feel as a cool draft. (this is why many people do not like heat pumps because of the cooler discharge air)

Normal discharge temps of equipment at the equipment. Remember we need to deduct a bit for temp loss(radiation) off the ductwork.

Gas heating temp range from 120-140 degrees, electric heat strip is 110-125 degrees, heat pumps are 95-115 degrees.

I hope this squared your question away.