This is why I try not to get a pro to repair things!
The fire will not light if the airflow is not goodenough tomake the "sailswitch close acircuit,sothat is notlikley the cause.
If the blower is running like you say, the fan that moves air into and out of the furnace is attached on the same shaft and it will be turning unless something has broken loose between that fan and shaft. IT "might/could" be loose but they tend to squall when turning on the shaft, so I'm guessing not that!
The most likely cause is simply the ductwork has come loose from the furnace or that ductwork is very stopped up in some way.
It looks like most of your ductwork is in the basement, so a check for hot air coming out there may be a quick check?
But a second item to check is that the air has to not only go through the furnace and out into the RV, but it also has to have a way to return to the furnace!
So take a look at the return path for anything that might be blocking it.
Any chance of a filter that needs cleaned or maybe something got stored in the wrong place to block the return air vent? Simple idea is that the furnace can't blow air in, if there is no way for the air to go out to the furnace!
There are three builds of your year and model of RV, so this may not be the correct one,depending on serial number.
This is the parts catalog where I got this drawing, so it may not match what you have exactly as I used the first serial number???
https://catalog3d.winnebagoind.com/menu/Parts.htm
But if we assume it is correct for this area of the RV, this shows the parts and airflow.
Cool blue in, warm red out?
Check a few of those and see if it shows anything?