Adding a second gas tank 1988 Travel Master

Maybe try to find one larger single tank that would fit.

If it's a Ford of that vintage, I think the option was a second tank mounted saddle style on the driver side of the chassis. Better, would be to find a single larger tank and see if its dimensions would fit. Our E450 has a single 55-gallon tank. Behind the rear axle so consider the weight distribution.

Also, you need to bear in mind the type of fuel pump, according to your fuel system. The carbureted engines use a low pressure screw-type pump. A late model tank will have a high pressure vane pump, too much pressure for a carburetor.

We had a 1983 Ford E350 with the behind-axle and saddle tanks. Behind was about 23 gallons, saddle about 19.

It is a Ford E350. There is no room to add a larger tank because the tank is saddled between 2 beams and left and right is occupied. If I add a saddle tank, it would only fit on the passenger side, over the exhaust pipe. I think that would not be a good idea.
The 2nd tank would have to go behind the first tank and there is plenty of room. I know from my manual that there was a 2 tank option. There was a switch that was thrown to view the fuel level and switch to the other tank.
This E350 has 2 fuel pumps. One is in the tank and the other near the engine. I can tell that the fuel pumps are designed to handle 2. I just am not sure where to go from here.

I need to get another tank, bolt it up, get a fuel pump to get the fuel into the system and put a switch in to turn on one pump or the other and send a signal to the fuel gauge.

We bought either the 87 or 88 new. At the time it was the largest in the line-up, maybe the "Signature?". It had the small gas tank as well. I found a 55 gallon tank designed for it. That was a long time ago and I don't remember the brand. I probably found an ad in a magazine and ordered it. I do seem to remember the tank may have "wrapped" around a frame crossmember towards the rear. I removed the OEM and replaced it with the 55 gal by myself. Several bottle jacks and lumber for cribbing it to lift the new one. I did have to get a new "O" ring for the fuel pump access ring. Never had any issues with it.
I would call every tank vendor you can think of and see if anyone remembers it.
edit add....we had to have a new modified black tank installed soon after the purchase. I can't remember the exact issue...maybe the toilet outlet pipe was too long...too close the bottom of the tank?? Might check that. It was a good RV. I saw one of the short ones at a NASCAR race last year.
I think ours was the last year for the C6 transmission. I didn't want to get the first year of the electronic overdrive. Fuel injected (I think) 460.

I had an early 90's f-150 that had 2 tanks. Each tank had a fuel pump and a fuel gauge sending unit. There was a switch on the dash for front and rear. When you flipped the switch it would send power to the other pump and sending unit. It was great to get close to E and flip it and watch the needle go to full. Although one time I was zoned out, and the engine sputtered, I hit the switch, and the other tank was on E also, luckily I was able to ease on into a gas station.

My current truck only has a 26 gallon tank and I get about 8mpg pulling my TT. Really 180 miles is at least 3 hours of driving. At that point I'm ready to pee and stretch my legs, so it really doesn't bother me very often. Every once in a while we will be we going through a rural area, late at night, and you aren't sure what will be open, and I wish for a little more range.

I am aware that there was a two tank setup and it works as you described.
Since this post I have learned that all the parts are available to do it. It is just a little more pricey and complicated than I want to get into to duplicate.
So I just ordered the 2nd tank, pump and support brackets. It is a 22 gallan tank which will give me another 150 miles and insurance.

What I'm going to do is just put a switch at the dashboard to turn the second pump on and pump it into the primary tank when I want more fuel
What I don't know is whether it will siphon across by itself. If it does I don't see a problem and it will just make it a larger tank but we'll see thank you guys for your help I'll report back once I get it working