Crack in the fiberglass exterior wall

drill and use Bondo. do NOT use silicone for this type of use IMHO.
bumpy

I just saw a commercial for the Flex Seal spray advertised on TV it now comes in clear.

I remember way back when that drilling a hole at the end of a crack can help to stop it from spreading. Use a drill bit that is a little bit bigger than the crack.

For my cracks I use a silicone sealant and then paint over it to match the color of the siding. It looks OK but you know there is a crack hidden beneath the repair.

For an invisible repair-
After doing some online research I plan to open the crack a little with my dremel tool and then use Bondo Hair to fill in the crack. Sand it smooth and paint. The videos online show this type of repair to be invisible and holds up well. Bondo Hair is regular Bondo with strands of fiberglas mixed into it. You can get it at an auto parts store.

I'd put a quality cover over the your MH for winter to keep the moisture out and causing more damage. You'll need to take it in to an RV shop that does and know how to do major repairs. It sounds like you have serious stress crack starting.

Keeping water out is no big deal - simple application of dicor or pro flex caulk should work on temporary basis. Keeping crack from spreading and final fix maybe bigger challenge, no experience with that but I remember reading a few threads so forum search feature might help.