Many people have had good experiences with the RV Water Filter Store, I would give them a call...

It shouldn't matter what type of connections the water softener has, just go to a hardware store and put together an adapter for your water hoses. If you have place to put a standard house size water softener that might work best if you're not moving anytime soon.
BTW what's an indoor rv park?

bjoyce and folivier, that is the exact one that we are in. I was told it was the only one like it. It's a very good concept for the harsh weather in North Dakota. In the winter, the inside of the buildings are usually kept at around 55 degrees. I really need to get some pics and post them, since the website really only has pics from when it was being built 2 years ago.
Also, I bought the WaterBoss 900. Its a residential unit but small enough to fit in the basement storage for travel purposes. I figure if we ever end up somewhere outside again, we can fashion some sort of insulated/heat taped shell to place over the unit and insulate the hoses of course. Not too worried considering where we are right now and we'll deal with the other when the time comes. Just seems to be a lot less hassle than the cylinder manual regeneration ones made for rv's. Plus, the new one is a 34,600 grain unit and the rv one is only 10,000 grain. With the extremely hard well water here, the higher capacity is much better. We can set the new unit to automatically regenerate as often as we would like at whatever time of day we want....regeneration takes about 21 minutes according to the specs online on our particular unit. Can't wait for it to get here in a couple days! Looks to be a very simple process to adapt the fittings to fit water hose. The only thing we have to figure out is the hose that needs to go to the sewer for the regeneration water. It needs to stay connected and include some sort of check valve so the sewage smell doesn't travel back up the hose into the softener unit.
I will definitely try to get some pics of the park up for everyone who is curious to see.

From my experience, the harder the water the sooner it will need to regen. More and more parks are against RO systems, they waste too much water. Figures I've seen say the ratio is 5:1 on those systems. It takes 5 gallons of water to make 1 gallon of soft water. The other 4 gallons go into the sewer system. Bullhead City AZ. had terrible water and we bought bottled water for drinking and cooking. We use a Mark8000 softener, but like you, found the hard water in some locations too much for the softener to manage.