It looks to me like those are stress cracks from flex in the area of the crossmenber, possible from rough roads or sway . It doesnt look like its a hard fix but its going to take some time jacking it up and getting everything back in place . I would suggest a long plate maybe 1/4" x 6" high and 3' long over the area . then adding a wider crossmember from side to side or gussets to the existing to spread the load out .
if it was rough roads that caused this I would suggest adding dexter equalizers that have the rubber in the middle , it helps stop some of the harsh bumps from getting into the coach frame. I would also suggest 4 wheel shocks on the axels , it does make a differance on the ride.
2001 Holiday Rambler 31' cks 10000 lbs loaded
2006 2500hd duramax/allison crew cab