Hi,
I just purchased a 1974 double wide, 24 x 70 and the center beam in the ceiling is sagging. I found the problem when I was measuring for new kitchen cabinets. th floor plan is open about 30 ft for the livingroom, kitchen and dining room. The floor to ceiling measurements at the outside wall is 95 1/2" but in the center its 91 1/2"; 4" difference!!! OMG. I think have an idea of how I can fix it but I'm not sure since I've never owned a manufactured home before. What I think needs to be done is to bring in a jack using a 4 x 4 and lift the ceiling very slowly, maybe a 1/2" a month until I've raised it back to level. Once I've done that a steel post needs to be put in place at about the half way point to keep it from sagging again. Does this make sense or is there another way to fix the problem? I'm looking for any suggestions that will make this repair as simple as possilbe. Thanks

Before you go jacking and installing support columns a few things come to mind.

Check the roof ridge from above. If the ridge line looks straight and even you don't want to raise one side and leave a step at the center of the roof. You will probably find that the sections are lagged or spiked together across the roof ridge, under the ridge cap. If this is the case you don't want to jack one side without the other as you could tear the roof sections apart causing considerable damage and a possible view of the sky

Homes built at that time were generally installed with support under the main frame I-Beams. They did not do much perimeter or centerline blocking. I would start at the bottom and work up. Check to see if the home is out of level, especially in the area that you see the problem. The way double section homes were installed in the 70's was most often to get one side level and then rack and force the other side to match at the floor line, so the home may very well have been out of square since it was installed.

A few additional piers along the center line or a full relieving might correct your problem.
Here are a few online resources that may come in handy-

Is Your Manufactured Home Unlevel?

How Level Is Level?

The Manufactured Home Releveling Kit

If it turns out that you do need additional support under the center line from inside do you really want a few steel posts on the middle of your home?

I would consider beefing up the interior ridge beam and adding support at the ends to maintain the open span. Perhaps two 2x8s the length of the span nailed together and stood on edge, with 4x4s or the metal posts under the ends. If it comes to that bring in someone that has experience opening walls for large window spans to figure out the best support needed to maintain the open span and where it is best to put the supports, to be sure the weight is transferred to the foundation system and not straight through the floor boards.

If you have a 30' clear span at the marriage line, your home has most likely been substantially remodeled. This is a much larger span than manufacturers typically offer. You will likely need to add a post with a pier under the home to support the post. while you are at it, you should make sure that you have piers under the home supporting both ends of the current span. Even if you live in area without snow, this is a significant span and must be supported properly to prevent this kind of sagging.