I know this doesn't fit exactly with RV's, but it's close.

I've got a 1975 Johnson 50 that leaks water into the lower unit. After a few trips the fluid is milky. I regularly remove the lower unit to change water pump. But how hard is it to replace the seals? I have a factory service manual coming from eBay. It's cool that I found one! There are 2 seals. One on the shaft going to the power head and one on the prop shaft.

I've always been frustrated by the fact that these older outboards don't have a pee stream to see that water is circulating. Kinda like old men. But I found a mod for that. Drill out the head for a $3 fitting, run some tubing to another $3 fitting in the lower power head case and you've got a pee stream. Both fittings on eBay. I'll need to remove the head to drill it, but that will allow for resurfacing anyway. Worst part is removing the 8 head bolts without breaking any off. Outboards get

A white film on the treads that make removing bolts risky.

Our marina sent outboard work to the respective dealerships.  However, outboards & I/Os were, in our opinion, a problem in the making.  Wear on the output seals invariably leaked salt water into the lower gear case... especialy with the inevitable bit of monofilament.  We did build a adaptor with the right threads...  add an O ring...  a quick connect...  & a small manual pump to fill I/O gear cases from the bottom.   

 

Usually the bolts are stainless but the gear cases are aluminum...  all suspended in salt water.  I'd try vinegar to dissolve as much salt as possible.  Hot water pressure is not a bad idea either.  I liked to use anti sieze wear ever possible...  but I avoided such things as much as possible.    

It has been about 12 years since I wrenched on outboards but I don't recall it being that big of a deal at all. 

Shouldn't be too hard to replace a seal if the bearings and gear set are ok. Need to replace the water pump while it is apart as well. I have never gone into a lower unit but have dropped several off to replace water pumps mine included.

Top and bottom have bearing/seal carriers. Bottom has to be pulled out and has threaded holes to accommodate the puller. Top looks easy. So no need to get into the gears to change seals. Looks doable.

I did one on a Force 50 a few weeks ago. Get you a seal kit from ebay or something, and all you should have to do is pull lower unit like you do to change impeller. Their is a shaft seal under the water pump and a o ring and another seal in the bearing case if I remember correctly. No special tools. Hopefuly your bearing retainer is not seized, or you might have to get creative and use heat and a puller. Also make sure you get the small washers that go in the fill and vent plug. other than that, fairly easy job