Build an a-frame?

I have a new catamaran that I built fairly recently

Which design> I love the ones Dudley Dix offers but they are behind my mean$

lot of "established" parks do not allow home-built campers or RVs of any kind

These might be the same parks that frown upon vintage RV's as well, an age restriction, which I may face if I purchased used. Thanks though, I was unaware of this. Hmm, so if I use a similar font and adorn it with the word "A Li er" rather than "A Liner" can I be accused of anything but poor spelling?

bathroom is IMPERATIVE

So you've met my DW? This is tricky. In backpacking groups privacy is often something that is requested rather than expectedWhen I walk the 20ft behind the lean to to the thunder box it is helpful if I let people know so they do not wander to close. I am thinking a small cassette toilet used either inside or one of those shower tents. Not ideal but it is easier to adjust one's expectations than it is to adjust ones tow rating.

Aliners are very cramped, even for 2 people

I'm 5'9" and DW is taller. The A frames actually seem to offer a nice inside vertical clearance. Open and airy. She spotted right away though - where does all of the stuff go. Not much space. Not ideal but it is easier to adjust one's expectations than it is to adjust RV Budget. And you are right, I am not disagreeing, other than teardrops I think they are the smallest RV out there.

You will be much happier with electric brakes

Very cool. I tow small boat and utility trailers so the whole braking thing is new to me. My van stopped my 1750lb boat fine but I figure brakes would be finer. It is easier to adjust one's stopping distance than it is to get an exception to that basic law of physics concerning two objects occupying same space at same time.

roof leaks

I envision ridged metal roofing one might find on a cabin in the woods. They even sell it in an arc so the roof "A" frame could be bowed out a bit to expand space. I would not pierce this with skylights or vents - those would go on gables (i think). But I also envision using the boating building skills to use thin marine ply sandwich panels with a foam core. Epoxy / fiberglass the skin. Treat the roof like a hull.

marine plywood (BS1088) is very expensive

Remember. Size matters. For an A Frame the roof panels are no wider than 8' and not much longer. I'd invest more in the roofing system than the sound system.

would a teardrop trailer work for you?

Alas no. Ability to stand inside seems to be a key factor. Thanks though - a lot of what I am thinking comes from DIY teardrops.