I bought a truck scale off of eBay.
These are frequently advertised as wheel scales, axle scales, wheel weighers, etc. Most of them these days are electronic, but the MD400 is a mechanical one that you see from time to time. I paid less than $200, they are heavy and it cost about $50 to ship. It is readable to 20 lbs, claimed absolute accuracy is 50 lbs to 5000 lb load, then 1% of applied load to max capacity of 20,000 lbs.
With only one scale you need a wheel shim of the same thickness so that the axle being weighed is level. There will be a very small error if the other axle is not also blocked to the same height, but that can be safely ignored.
The results for my '99 F350 DRW 4x2 with Bigfoot 10.4E:
LF 1930 RF 2160 total 4090 GAWF 4550
LR 3500 RR 3640 total 7140 GAWR 7460
total 11,230
GVWR 11000
This is with nearly full tanks (fresh water, propane, diesel), full owner's outfit, but no clothes, food, people. As you can see I am just under on each axle (but will be over on the front with 2 people and the cat), and over on GVWR. This is a relatively light camper, no slides, no air conditioner, no genset. The "owner's outfit" I transferred from the old camper, about 4 armloads worth - I doubt it was more than 100 lbs.
I remain truly amazed at those who can put a 2 slide camper on a 3/4T pickup and keep in under GVWR - filled with helium maybe?
You can weigh the camper directly with the scale. By measuring the weighing points, you can calculate the CG too.
Bigfoot 10.4E, 2015 F350 6.7L DRW 2WD, Autoflex Ultra Air Ride rear suspension, Hellwig Bigwig sway bars front and rear