When my furnace is running I can touch the hot box with my bare hand. I do not think this is normal is should be hotter. I have a flame, the fan works but it is just not putting out heat. I am in a manufactured home and have a coleman furnace the year built was 2001 Shult homes was the builder. I had a furnace guy come in and tell me that I need a new one because mine is a 56000 BTU and I should have a 70000 BTU

The size of your furnace will depend on the size of your trailer/mobile home and the local weather situation. The difference between a 56 and a 70 is not that many BTU. I would have your unit checked over it is possible that the orifice is partially restricted OR the Gas regulator is adjusted improperly or in the case of oil the nozzle is the wrong size.

This will cut down on the capacity of the unit and make it seem to run cold.
Call a real professional to look into the above before you replace.