Electric brake controller... seriously, if you don't have one buy one, if you can't afford one, break into your neighbour's and steal his.
For your 20" tire question, your towing capacity will only be affected by how much weight you put on the tires and what their max pressure is. If you have Load range E tires, you can put 80psi of pressure in them from weight but by then your suspension should be sagging like an old granny's .... so if you have issues with sagging, then you can get a helper bag kit.
My parents had a 27' 5th wheel trailer that their 5.9L gas pulled alright, but was still slow on hills. Anything heavier you'd want to look at a diesel for sure.