if you wanna do a winter weekend or up to a week in the summer, fiberglass topper is perfectly good. if you wanna spend months on the road at a time, get a slide camper.
if slide in and youre on a budget, just find one on craigslist or offerup. you can find them for free if youre down to do the remodel. most of the time you need to gut it and get all new appliances, walls, cabinets, floors, vents/fans, lights, water pumps. it could be as little as $2k. just make sure your wet weight doesnt exceed gvwr
if slide in and you can fork out $20,000 then get a scout. ive driven with my friends tundra towing a sled trailer and i could hardly notice the extra weight from it. they are dope but dont have a bathroom integrated. he works for ministry of natural resources and forrestry and goes offroading on gnarly logging trackswith the camper on. lives in it full time.
https://scoutcampers.com/kenai/
if fiberglass topper, i have a ranch and honestly its totally fine. great value for money. it doesnt fit/look perfect like a snugtop or ARE, but good enough that you dont have giant voids. the paint match is perfect, but the overall finish of ranch isnt amazing. cab hi are not very roomy, but really warm in winter. the high rise ones just dont keep the heat down imo. mid rise is think is best. unless you want to build a giant box to put your bed on, then go with a hi rise.
https://www.ranchfiberglass.com/truck-caps-and-tonneau-covers/skyline-truck-cap/
-get the headliner, it soaks up condensation well.
-get a side opening window, a side slider and the cab slider. you can put a towel to seal off into the cab and i usually keep my cooler on one of the back seats and my duffel bag on the other so i can reach in from the bed easy.
-for heating i mounted my propane buddy heater on the bed rail right by the tailgate and i just turn it off after it warms up in the bed. you can go the chimney and stove route, to avoid moisture and carbon monoxide hazard, but you dont need to. just get a CO detector and fire alarm just in case you fall asleep with it on.
-you can pull 12vs from the trailer harness all the way at the back for a power inverter.
-get a good matress to cover the entire bottom, thats really all the insulation you need. those air matresses dont hold up for winter camping. a heater fills up the area with warm air really quick. i dont have to run mine for more than 10min. worst ive been in was -25 celsius and i only ran the heater in the middle of the night once.
-get a rack and ski box. you dont want to be sleeping with your wet stuff. i have two sets of intuition liners gloves and mid layers that i swap out when they get dank.