Top grade drybags are heavy and your baggage can start to add to to a considerable weight.
I use lightweight ripstop nylon drybags which are fine for rain and occasional splashes but maybe not up to canoe style extended dunkings. I line my worn out Carradice pannier with a drybag and all my clothes and electricals go in one side.
When I can't put my sleeping bag in the pannier, it goes in a compression sac, inside a drybag with the small sleeping mat, on my racktop. If you can find a compression drybag that may be better.
Tents get wet and should not be inside dry bags. My tent came in an oversized sylnylon bag so it goes in wet. The bag is highly water resistant but not immersion proof.
Many tourists use 2 panniers and a large ortleib drybag on the racktop. personally, I think this weights the bike rearward and it forces you to put wet tents inside drybags.