RMH for a small space starting with a water heater core...

I had arrived at this forum via a link that suggested discussing the RMH stuff, and knowing no better, I posted my query here:
my query
Apparently that was not the correct spot, so I would like to continue it here, and hopefully get some good info as I'd like to get this well underway ASAP !!!

The quick background:
I live in a teeny-weeny old camper with zero extra space except on the outside of it, between the camper and a big tent built right over it.
There's a wee bit of space beneath it, so I'd like to run the exhaust tube from the RMH under it and out the back flap of the tent.

Here's where this project stands right now:
- I've made the space beside the camper as ready as I can.
- Scrounged around for materials
- Searched quite a bit for construction details.

Materials on hand:
- A small stack of bricks
- An old propane fired 30gal. water heater carcass.
- A used-up ~20 gallon water expansion tank

I have a wirefeed welder and 4" grinder so I can do some fabrication work.

Looks like I'll need to buy some stovepipes, some kind of exhaust tubing, mortar and perlite.

And there's ALOT of sticks & yard waste around here that will burn nicely...when I can get it all done.

I've seen someone's idea of using 4" stovepipe for the inside/insulated flue and 4" flex tubing for the outlet.

I'm wondering these things, if anyone can help me out here:
- What is the practical length one may extend the outlet tubing to ? (Is 20-30' easily do-able ?)
- What size pipe is usually best to surround the 4" flue with so as to hold the insulation ? (6, 8, 10" ?)
- What advantage of using tamped-in perlite vs. the used fiberglass from the old heater ?

I can butcher up the old water heater any old way - even re-use it's built-in flue if that is a good idea, but I'd like some suggestions as to how best to do it because soon enough it'll be snowing regularly and I'd rather have it ready than to be still learning via trial & error...

Having read a bunch here, watched videos, and checking out a bunch of sites mentioned here including Len's Rocket Site has given me alot to go on, but it is the things I must buy that most concern me right now.

Thanks.

May be some ideas to glean there.

http://www.youtube.com/watch?v=IjTIsiBbsk0&feature=relmfu

I'm amazed.
Of all the possible places on the internet I'd sort of think of this one as the home of the rocket mass heater - OK, so...
I posted here hoping for some help and/or guidance and...and...and...nuttin after over 2 weeks.
Wow.

I've read 'the book', a couple of times, and have some clues & ideas of my own, but sure would appreciate getting useful, usable, maybe even tried/tested ideas from others.

I simply cannot imagine there's no permies a'tall who live on a shoestring in a tiny place like I do.
Living small is so wonderfully efficient, and it takes so much less in order to do it !!!