Did vents get installed when they built it? By code there needed to be vents at least every 10'.
The best ones are automatic opening, Then there's no need to have to open of close them. There about $20.00 each and sold at any box store.
The trouble with adding a dehumidifire is who's going to go under there to empty it? It would need to have a hose added and let it dump into a sump to be pumped out, or a condinsate pump added. Everyone I see is put under the house and no ones ever gone under the house to check on it. By the time they do the control circuts is so corroded it's not working.
For a dry crawl space the grade under the addition must be higher then the outside grade, if not fill needs to be added. A simple layer of 6 mil. black plastic over the soil held in place with 16" insulation ties bent in half will hold it in place. There must be ventalation. most codes want the vents to be 12" above outside grade.
There has to be working gutters or the waters just going to work it's way under the house. Thee can not be mulch piled up againt the house or it will hold in moisture, Grade must run away from the house.
Your going to have to trust me on this one, at least 80% of the work we do is on 100 plus year old houses and we have been under 100's of homes and have seen what works and what did not work.
I've seen companys come in and cover all the vents with foam, tape all the seams in the vaper barrier, run it up the walls and nail it in place with strapping, add a dehumidifier. But they never condisioned the air, never raised the grade, never at least fill in the low spots, never added gutters. There was so much water left laying on top of the plastic due to bad grading and mulch againt the home that all the floor joist were covered with black fungus and looked like they had been painted with black spray paint.
What a lot of insurance companys used to do with old ballon constructed homes is make them completly seal up the foundations because of the fire hazard with that type constrution. I had been under the homes before and then years later after it was closed up. Every one of them now had fungus growing because of the lack of ventalation.