What kind of mods for a camper?

[VileZambonie]

The Th400 is the strongest most durable of the pack. The 700R4 and 350 are about the same on the durability scale. Heavy towing in overdrive is not recommended but havinig overdrive is nice especially if you have 3.73's or (numerically) higher. Either will require upating and modifications and as someone else mentioned the use of an auxiliary oil cooler

[werewolfx13]

Personally, I've never seen a 700r4 hold up to anything better than a stock 4.3 for more than 100k miles..I've got a dissassembled th350c in my garage that was only retired because the pump died @ 277k miles, and took the converter with it.

Use an external cooler no matter what you go with..a 4 core radiator is also a wise choice. Connect the external cooler in series with the trans cooler built into the radiator..run it through the external cooler first, then the radiator. This is especially wise in areas that see cold weather. Use of an external cooler and regular fluid changes will greatly extend the life of ANY transmission. If you've got money to burn, a good aftermarket overdrive can be used to give you the strength of a 350 or 400, with the improved mileage of OD. Gear Vendors makes a pretty good overdrive.

[VileZambonie]

For less than $250 bucks you can make the 700R4 very durable. I built one 6 years ago that sits behind my buddies 84 K5 with a 454. He tows, burns tire, and daily drives with it. 700R4 upgrades are really simple. Sunshell, pump & vane upgrade for older units, PR & TV boost, Servo, Shift improver etc... I've built so many 700R4's I've lost count. I'm doing a 4L60E now and a 700R4 that will both be strong and reliable. they got a bad rap from early model units not being able to take much abuse but with the proper treatment they're a better alternative to the 350.

As far as the oil cooler goes an auxiliary cooler should be installed AFTER the radiator cooler on the return side. So connect to the top outlet at the radiator and then go into the aux cooler and back to the trans. The only time you would want to install it b4 the rad cooler would be EXTREMELY cold climates. Trust me on this I am an ATRA R&R, Rebuilder, Diagnostician & Proctor.

[DirtyLittleSecret]

Zambonie has it right.  The EARLY 700R4's had alot of issues, but can be overcome by any good tranny shop.  Later 700's, and 4L60e's have proven themselves excellently.  Either way, the TH400 is the stoutest brute of the bunch, and the TH350 can be beefed up to compare with the 400.  Unfortunately alot of shops will cut corners with the 700r4's ($$$).  A TH400/350 can be beefed for alot less money (often still a cost benefit in thier favour if total replacement is happening).
Personally, if it were me I'd look for a more appropriate rig (3/4 ton/1 ton) that's meant to do this. 
A rig that can be made to do something, and a rig that was meant to do something are two things that are different.

[werewolfx13]

I may have to consider building an r4 to go with the 350 I'm rebuilding for my truck then..I had planned on rebuilding my th350 when the time to swap came anyway..though I'll need to get a manual to guide myself around the r4..About how much longer does it take to build/rebuild a 700r4 compared to a th350? I can tear down and rebuild a 350 on the bench in about 6-10 hours, if I've had the new clutches soaking in fluid overnight and don't run into any problems..like finding out my clutch press, snap ring pliers, and torque wrench have been ganked. 

[VileZambonie]

Well you just named basically all the tools you'll need with the exception of the seal installer for the low reverse clutch piston. I use a plastic sheet of paper, rollit into a cylindrical shape and put scotch tape on it. Buy the ATSG book and you'll be glad you did.

http://www.google.com/products?hl=en&q=atsg+700r4&um=1&ie=UTF-8&sa=N&tab=wf