We had a large pop-up back in the 70's and it was pretty amazing. Certainly no problems with rain or even snow entering it and its furnace kept the rig toasty warm in any whether. Todays pop-ups are even more amzing, with more amenities, including even small slide-outs! And most have a solid roof and synthetic material where the "canvas" sides used to be, so drying it out is not so much of an issue (though it will still mildew, inside and out).
You might also consider one of the "hybrid" styles, with either folding hard top & sides or the travel-trailer like bodies with pop-up like push-out rooms at the ends. Both of those are substantially lighter than a regular TT with a fixed length equivalent to the unfolded dimensions of the hybrid. Google hydrid rv or hybrid camper to see a selection - just about all trailer manufacturers offer the TT-like hybrid. The folding hard body style is more specialized - see www.trailmanor.com for an example.
I think the hybrids may be more prone to leaks than soft sides models, though. Perhaps it's harder to form waterproof jonts with solid panels? Obviously it something to be considered when looking at the design of the individual units.