Addition of the camper changes things somewhat.
The SM318 and T5 are rated for only about 6000 GVW and might not hold up satisfactorily in the long run with the camper. The A833/MY6 overdrive 4 speed is quite a bit stronger and would be a good choice if you don't need a 1st gear much deeper than you have now with the SM318. If you need a grannny 1st gear, you would want something like an NV4500. This is a truck 5 speed with a granny 1st and an overdrive 5th.
The A833 is a relatively easy install. If you get the right one (next paragraph) it will bolt directly to your stock bellhousing after you open up the center hole to 5.125". This is a simple job that any machine shop can do. It is the same length as a "long" SM318 (about 21" from the bellhousing face to the rear seal) and has a 27 spline output. If you have a long SM318 now, the most you will have to do is change the yoke. With a "short" SM318 (about 16"), you will need to shorten your driveshaft.
The one gotcha is that the GM version of the A833 was made with two different bolt patterns and only one of them will work with your bellhousing. You definitely need to take a look at what you are buying before you lay down your money. The one you want has a normal GM bolt pattern that looks like the one on your SM318. The transmission you DON'T want looks like the one on the left side of this pic:
http://i98.photobucket.com/albums/l2...ver/NP440b.jpg
The transmission on the right has a normal GM pattern, but that one is a Muncie, not an A833.
If you want to use an NV 4500 there is no version which will bolt directly to your bellhousing. Fortunately, Advance Adapters sells a plate that will let you use a Dodge NV4500, but not the GM version.
http://www.advanceadapters.com/produ...ter-plate-kit/
You will still have to open up the center hole the same as for the A833 and also modify the driveshaft.
Ray