Air conditioning in mobile homes is usually installed by an independent air conditioning contractor, although some dealers have their own crews to do this. Service calls about water in the heating ducts are not uncommon and mean the drain for the condensed water is not working properly. If you have taken the time to keep a record of who did the AC work you will know who to call. Note that for those in dry climates, air conditioning is a completely different matter from evaporative (swamp) coolers.
Especially if you live in a hot climate, old air conditioners will probably cost you more than they save. Newer units have much higher efficiencies. Old units wear out and cool poorly for the same power cost. A replacement may be expensive at first, but it will likely pay for itself.
One very fixable failure is if the compressor fan fails. This is pretty common. As the fan ages, you may notice that the air conditioner kicks on frequently and doesn't seem to cool well. As the problem worsens, you may hear a 'clunk' sound from the compressor unit when it turns off. This is the thermal overload switch shutting off the compressor before it overheats even more. Ignoring a failing fan for too long may leave you with no choice but to replace the compressor as well.
Window units may also provide an effective means of cooling a hot end of the house. They are relatively inexpensive compared to a system for the entire house, but you will want to consider a few things when using one. Condensation is one issue. Make sure the drain is clear and that it drains safely away from the house to prevent water damage. Also, these units can pull significant amounts of power. Adding an air conditioner to an already-loaded circuit may trip a circuit breaker.