Flood plain is a very complicated issue. Question #1 is if the flood plain is accurate. The Corps of Engineers is always changing the flood plain, so make sure that you have updated confirmation that you are in it – not something off the city hall map dated from 1964. If you are in it, the next question is what the worst case scenario is. If it would not impact any lots or roads (for example, just a back corner of the property which is currently a playground, then the impact is slight. If, however, it could wipe out the bulk of the lots, or shut off your main road, then that could be a deal killer. As for financing, every bank is different, but they will also be looking at the worst-case impact. As far as new development, it would have a dramatic impact on that, as most mobile home parks are grandfathered, whereas new construction would require new permits and it’s unlikely a city would allow you to build a structure in the flood plaiin.
Another thing to look into is what it would take to get the park out of the floodplain. If all you need is a foot of fill dirt, then maybe you can fix the issue. If it’s a 50’ earth retaining wall, then probably not.