Fifteen years out, it is good to be thinking about the financial and health questions, but probably too early to be much concerned about what type of RV, or even whether RV travel, let alone work camping, will be your retirement mode.
I retired early, laid off at 58 after a merger and business downturn (i.e. oil price collapse). My slightly younger wife wanted to keep working, but shifted to part-time so that we could travel about six months out of the year. Part of that was international, part of it RV travel. That worked great for four years, until she got sick shortly before her minimum retirement age. The next three years were about managing her illness and working on her bucket list, not a lot of that reachable by RV. She lived ten weeks past reaching the "normal" retirement age of 65 (though Social Security Administration has already bumped that number a couple more years).
More cases, in our RV club, most of the members have been camping or RVing since early in their life, age 30-40, and continued until their physical limitations meant that they just couldn't do it anymore. For some, that was not long past age 50, others have done fine into their 70s and 80s, we have one 93 but she needs a lot of help.
The limitation that tended to bring an end to RVing in out club (when it was not dementia) was usually not being able to get back up from the work that has to be done on or close to the ground. That does not really go away with a motorhome, somebody has to get down to do the hookups, maintain the tires. The stairs can be overcome with lifts, or with ramps to the door of lower RVs.
You don't really know what is 15 years ahead. If you want to get a diesel pusher, just to try a diesel pusher, and you can afford it without compromising your retirement financial plan, I suggest you do that now.
In fifteen years, we could have low-slung Euro-style RVs, just one step up to floor level. Most people would find them too small for full time living, but I have met people who full-time in tent campers, van conversions, and TTs as small as 13 feet. There are a lot of possibilities.