Yes, many RV owners have successfully set up CARB HDIM accounts, but the system has known glitches. Your account becoming invalid after verification is a common technical issue that requires specific browser settings and persistence with CARB support.
Best time to buy used Class A diesel is October through February when dealers clear inventory and fewer buyers compete. Worst time is spring when demand peaks. Start shopping in fall for best selection and deals.
RV engine replacement is possible. Options include rebuild ($5-15K), reman ($8-20K), or used ($3-8K) plus installation. Compare total cost to RV value before deciding.