We did exactly the same thing you did - retire to La Paz, BCS, Mexico in our 5th wheel. Here's what we do and it seems to work well:
1 - we carry things in our 5th wheel that look like they belong in a 5th wheel as part of our furnishings.
2 - if we have extras - such as laptops or electronics, we always take them out of their box (if they are in it) so they do not appear to be new items that we need to pay a duty. Sometimes, we put them under our bed which can only be accessed by opening our slideout. Even when we have had to open our slideout (once) during a military inspection, this has been no problem.
3 - We cross a Tecate - a smaller crossing and we have never been pulled into secondary for an inspection there - just waved through.
Obviously, you don't want things to "stand out" as being extraneous to your RV, but since most Mexicans don't know what is really part of our RV and what is not - we've never had a problem.
If you would like to send me a PM or email, feel free - since we may have some things in common with you and can carry this discussion on that way to help you out.
Remember that the $50 limit is what you can expect if you are flying. When driving across, the bare furnishings of your RV are clearly going to be above that - but we've never had to pay a duty.
If you have a lot of extra stuff that clearly looks like you're moving, then it could be different - but we've always integrated our stuff into the RV and it's been fine.
Of course every year that we cross back after visiting in the US for the summer could be different - but so far that's been our experience.
A couple of good Yahoo groups that you might want to subscribe to for La Paz living are La Paz Gringos and La Paz in My Opinion.
Good luck and feel free to PM me if you like!
Charlene
