LENGTH

Length will be new truck length - 8' (for bed) + camper floor length.

OR

new truck length + length camper extends past the rear bumper of the current truck.

Joanne, also post what pickup camper you have. as well as what your current truck is. Some newer trucks have higher cabs and some old campers will need to be raised in bed of truck to clear cab.

Discounting my previously snarky comment (injecting some humor only), you need to ascertain the bed length of your present TC. Is it for an 8 foot bed or a 6 foot bed? An 8 foot camper will fit in a 6.5 foot bed but will hang out the back quite a bit and my put the Cg in back of the rear axle which may impact steering and handling.

Another aspect to consider is, new truck manufacturers are changing things like tailgate width (openings) and cab height (higher) so your old unit may not even go in a new truck (depending on what you buy or the cab over may not clear the cab roof without some modification (spacer) in the bed. I'd take measurements of your old unit and have them along when shopping for a new truck.

Not sure if I am understanding the question. If you are looking at a new truck the dimensions are easily found online. If you are keeping your old camper measure how much extends off the back off the truck and add that to the trucks length. That should get you pretty close. The actual overall length may be a few inches shorter for the bumper which would be included in manufacturers length.

You compute the length of the truck by your ability to pay for it....