Assuming you have a competent park maintenance man… not a crackhead
Windows you should be able to replace window for $200 apiece. I have ordered fixed glass vinyl windows 16"x24" from Home Depot for as little as $37. This window was for a door. With most window I have seen being surface mounted install is less than an hour labor.
Paint with an airless paint sprayer should be a relatively quick two man job. Figure paint to be 50% of the cost. I have even used cans of spray paint to touch up older wind blasted white trailers.
Doors. Depending on if it’s a newer trailer with standard construction you should be able to buy a new exterior door for under $300. Plus labor
Roofing. Snow sealing might not add sellable value but as you stated water leaks wreck everything and are hard to track the true source of the leak. I have found metal roofs to be about twice the cost of 30 architectural shingles. Be careful on if you have your maintenance man do the roofing. Workman comp insurance can gets expensive.
Interior. Rugs (flooring) and paint. It will get rid of the “Smell” be it cat pee or just the previous tenant stink. Plus both can easily and quickly rehab a space. I would also replace any outdated light switches (.50), plugs (.50), covers ($.18) and light fixtures ($10). Counters, sinks, faucets can get expensive. Craigs list is a first source for appliances but always compare them to new low end Home Depot units. When you need an appliance quick, or can’t find what you need you can always pick up a new low end Home Depot $350 range or $600 refrigerator.
This all being said you must always ask yourself if the unit is worth rehabbing. If it is a 1970s unit with any structure issues the chances of being able to fix it up cost effectively and attract quality tenants will be challenge to say the least. At the park I work at, that we are turning around, we just had to make that call twice. It is hard bite the bullet of paying to haul off a trailer to the dump and then spend upwards of $25K for a newer trailer and then find a buyer with down payment and credit when you can rehab an older unit for $3-5K and rent to own over 18 months.