oh i think its pretty dam umm i mean darn strong.
now on mine i have the 2" foam, then a layer of full strenght glue, then the canvas. on top of that i have a coat of diluted glue. then a coat of primer and 2 coats of paint. that is the outside.
the roof is the same except for the thickness of foam which is 1" there.
i have the spars running sides ways. i will have to go count how many i actually have. also there is a piece of wood that runs front to back. the purpose for that was just incase someone leans on it, that foam over time will not go anywhere. and a bulkhead 1/2 wall.
i have leaned full force on it, i have given it a few good wacks. but i have not taken a hammer to it and i have not stood ontop. but i have tought about that as i was painting the roof. it was solid.
if you do your seams right no water is going to get under them. mine are not going to leak. but they aren't pretty. so all in due time, cosmetics will take place.
i have a bunk bed in there for a child. hubby is afraid though that they might kick out in their sleep and put a hole in the foam. right now, he might be right, because the inside is unfinished. but once it is finished i will not worry about the kicking issue.
but i do think i could get up on top of the tear. i just don't really want to test that.
and to ease my mind about that darn hatch, i am going to use the cinch straps to run front to back for traveling. we still have some kinks to work out. but it is coming along. now if we made a second one, i would still use the foam. but there are things i would do differently.
1 and this is the most important: take more time! don't rush it. make sure all pieces fit together properly.
2: i would precanvas at least the inside walls and roof.
..i think the outside should be done once its built. that helps to get that nice tight fit.
3. would use a wooden frame for the doors. which we are going to do that eventually also.
now some things i would not do differently:
i would not change the way i did the layering of tools. i would still go full strenght with the glue on first layer, i would still use the dilution and primer and paint.
but on that painting issue. if you want a design, i think you need to prime it and not put down anycolor where you want to add other paints. that did not sound right. i did mine in that solid reddish/burgundy color, the one thing i noticed now is that it doesn't take well to anything added on top. so to paint a design on there now is going to be hard. the paint or water or glue or poly just runs right off. that is a plus, but it is a negative for adding a design to it.
oh and another thing i would do is to build it a bit bigger. never any smaller, but for sure bigger.
you already know [i am sure] is that i am a worry wart, and that must come for me being a mother, the good mother, not bad mother.

but i am not the least bit worried that this will NOT hold up. that should tell you something about the strenght.
so be patient, take your time and decide ahead of time what your finished design/paints will be and work towards that.
now your td gpw, you already know about painting, and canvas so you will be ahead of the game. and i think you said you are not using the diluted layer, just going to the primer.