We've only been doing FT a year so far, with many years of long term travel.
As you can tell space prices vary a LOT. For example, we want to stay outside Seattle for the summer. If you can find a space, they go for around $1000/month plus electric. We usually end up in the KOA paying by the week ($1200-1500/month including electric). That is the most expensive we have had to put up with. Currently we pay $600/month which includes electric.
Propane depends on the trailer and weather. I went through a 30lb bottle of propane in three days when the lows were in the teens and high was low 30's. During the summer in South Carolina a bottle has lasted us over a month.
Electric is also season and area dependent. Southern NM our power bill was $150/month (running 2 AC all day, $0.14/kwhr).
Also check about tow vehicles. Some parks require you to have a tow vehicle on site that can move your trailer. They also require that MH's be operational. One park explained that it was due to any park maintenance requirements that would require moving the RV. We have been asked to change spaces when they had to dig up a water line to fix a leak.
Trailer. Whatever is comfortable for you. We (wife and I) picked a 35ft bunkhouse model. We use the bunkhouse for our hobbies. It has a workbench with his and her tools (weaving, quilting, modeling, reloading, etc). Also makes a really nice extra storage area.
charlie