Sadly, when it comes to W-C Skippers, Imperials and Juniors, that lifespan is less than half as long as it would have been--as it was designed to be!--if Thetford Norcold hadn't decided there's more profit in a frequent turnover of cheap disposable toilets than there is in replacement parts for top quality toilets.

You're the second person to ask if the PHII/PHC conversion will fit an existing toilet's base. You only have to look at the exploded drawings for your own toilet and the PHII to know the answer to that question is no. Then you need to read the "conversion" info again...it's only the BOWL that's recycled (along with the seat and lid if they're also in good condition. Everything south of the bowl is replaced.

You asked for a suggestion: if you decide to have a new shaft machined, buy a portapotty to use for the duration. Or.. swap your Jr out for a PH II.