Roof ventilation / repair issue

I recently purchased a older double wide mobile as a fixer upper for a rental. I knew when I purchased it that it had roof issues, and I was able to get it for a price I think will make it worthwhile.

The roof has significant waviness to it on one side of the mobile, and not nearly as pronounced on the other side of home. Having heard horror stories of 1x2 trusses on mobiles, I was very anxious to find out what the structure looked like to make a plan. This weekend I opened a small section of the roof so I could try to minimize the number of surprises when I start the project.

Here is what I found: The mobile has vaulted ceilings, and it has no trusses - it is single 2x6 rafters spanning 14ft 4 inches, spaced on 24 inch centers. The pitch is 3/12. The roof sheeting attaches to the top of the 2x6, and the sheetrock attaches to the bottom.... The baffling point is that I can see significant sag and wave in the roof, but when you look inside the rooms, it is not evident, nor does the sheetrock show significant damage or cracking....

One other wierd point - at the peak, the sheeting has a 4 inch strip of sheeting at the peak, and then the sheeting from the roof overlap's that, leaving a very small gap near the peak that the sheeting doesnt contact the rafter. My only guess is this was an attempt to create air movement.

My initial plan is to marry a new 2x6 allong side each sagging rafter to provide a solid base for the new sheeting (I plan to replace it all as it feels soft). Anyone have any suggestions on a different way to approach this?

My biggest concern is ventilation. I suspect the sagging and soft sheeting may be from poor ventilation. Each rafter section basically creates a sealed box with no ventilation. Currently their are 7 standard vents on each side of the roof, but their are 35 rafter, so alot of the roof is not ventilated effectively.

I was leaning towards a ridge vent spanning the entire structure, but where the two halve of the home joins creates a solid ridge of about 3 1/2 to 4 inches in width, and thus the ridge vent would need to be upwards of 6 inches wide to be effective. Is that possible - some people say you cant ridge vent a double wide, and others say it works...

If a ridge vent wont work, does anyone have any suggestions on a different approach for ventilation?

Also, I'd really like to add some insulation when I have the sheeting off, but I'm concerned it will just hinder the air flow even more - how much space do I need to leave for airflow?

Thanks in advance for all your input.

The double wide mobil homes I've worked on had undersized sheathing, no H clips between the rafters which can cause all those dips you see.

There is no fixing all those low areas without removing and replacing the sheathing and adding the missing clips this time or using T & G Sheating.
For a roof with low a slope I feel it would have been far better to have used EPDM not shingles.
It's lighter per sq. ft. it will reflect heat if you use the white material, far less seams to leak.
Not the cheapest way but it should last a min. of 25 years. That would save you from having to replace that roofing two times so in the long run it's cheaper.

I think the board under the sheeting at the ridge was an attempt for venting but it's above the vents so probibly didn't do much. Any time we frame like this we use 2x10 for lots of insulation and cross strap it with 1x4, or 2x4 for air movement above the insulation. If your taking all the sheeting off you might find mor problem than just sagging. At that point you may want to look at scisser trusses, that would leave the ceiling and give a 6 or 7/ 12 roof.