If rent is $300 for 6 of those months and $300 to store = $2,100 a year plus just say the DP lasted 10 years = $2,100 + $100 per month for DP = $3,100 a year. If you shoot for a 10 year time frame and you know that you can keep it on site for that long it's probably not too bad of a deal. The key thing that you mentioned is that it's your favorite place.
You could probably get a decent TT with a big slide for that much money. One advantage with the TT may be if they would let you remove the tires and lower it down 10 inches or so and skirt around it. I think the risk is lower with a TT if something goes wrong in the plan. Just about anyone with the proper truck can move it very cheaply vs trying to tow a DP that may have the brakes froze up. Towing a DP isn't going to be cheap.
I have looked at mobiles in Fl over the years and some very nice ones can be bought very cheaply but with $400 a month rent plus possibly keeping the ac on during the humid months I figured I was looking at $6,000 a year min. Then you have to consider the cost of insurance on DP or Mobile as some places wont let you store or park without insurance. Others will know more but isn't it susceptible to Fl lic or tax or something if it is there all the time?