By the calcs, your tires are about 33" tall. Tape measure to verify.
Personally I'd pick up a used, larger 17" tire for the spare. Somewhere between a 285/70-75 and 295/70 17 is what you're after. Plus if you find the right one, they are rated close to 4klbs.
Maybe not the comfort level you're after, but guys run these tires all the time under heavy campers. But as mentioned, in the unlikely event you have a 19.5 go down, your up sized 17" spare will work fine on the front for as long as you need it to.
Also, for slow leak, nail in tire etc, a tire plug and a small air compressor is sooo much easier than changing a rear tire with the camper on. Unless you're prepared for that with a heavier jack too.